SCons: Refactor Linux linker options with linker=<bfd|gold|lld|mold>

The new option is `linker` and lets the user specify the argument to
the`-fuse_ld=` linker flag directly. The supported options are:

- `default`: No change, typically uses GNU ld (bfd) unless the user or
  distro picked a different default `/usr/bin/ld`.
- `bfd`: GNU ld from binutils
- `gold`: GNU gold from binutils
- `lld`: lld from LLVM
- `mold`: mold, an extremely fast modern linker, not (yet) intended for
  use in production but great for development speed. Provided by distro
  `mold` package or needs to be compiled from source and installed to
  `/usr` otherwise.

Removes the `use_lld=yes` option, and make lld actually usable with GCC
too.

Not all the above are compatible or recommend for LTO, we recommend
using GNU ld with GCC LTO, or lld with LLVM ThinLTO.

platform/linuxbsd/detect.py

@@ -1,6 +1,7 @@
 import os
 import platform
 import sys
+from methods import get_compiler_version, using_gcc
 
 
 def is_active():
@@ -27,9 +28,9 @@ def get_opts():
     from SCons.Variables import BoolVariable, EnumVariable
 
     return [
+        EnumVariable("linker", "Linker program", "default", ("default", "bfd", "gold", "lld", "mold")),
         BoolVariable("use_llvm", "Use the LLVM compiler", False),
-        BoolVariable("use_lld", "Use the LLD linker", False),
-        BoolVariable("use_thinlto", "Use ThinLTO", False),
+        BoolVariable("use_thinlto", "Use ThinLTO (LLVM only, requires linker=lld, implies use_lto=yes)", False),
         BoolVariable("use_static_cpp", "Link libgcc and libstdc++ statically for better portability", True),
         BoolVariable("use_coverage", "Test Godot coverage", False),
         BoolVariable("use_ubsan", "Use LLVM/GCC compiler undefined behavior sanitizer (UBSAN)", False),
@@ -111,15 +112,32 @@ def configure(env):
             env["CXX"] = "clang++"
         env.extra_suffix = ".llvm" + env.extra_suffix
 
-    if env["use_lld"]:
-        if env["use_llvm"]:
-            env.Append(LINKFLAGS=["-fuse-ld=lld"])
-            if env["use_thinlto"]:
-                # A convenience so you don't need to write use_lto too when using SCons
-                env["use_lto"] = True
+    if env["linker"] != "default":
+        print("Using linker program: " + env["linker"])
+        if env["linker"] == "mold" and using_gcc(env):  # GCC < 12.1 doesn't support -fuse-ld=mold.
+            cc_version = get_compiler_version(env)
+            cc_semver = (int(cc_version["major"]), int(cc_version["minor"]))
+            if cc_semver < (12, 1):
+                found_wrapper = False
+                for path in ["/usr/libexec", "/usr/local/libexec", "/usr/lib", "/usr/local/lib"]:
+                    if os.path.isfile(path + "/mold/ld"):
+                        env.Append(LINKFLAGS=["-B" + path + "/mold"])
+                        found_wrapper = True
+                        break
+                if not found_wrapper:
+                    print("Couldn't locate mold installation path. Make sure it's installed in /usr or /usr/local.")
+                    sys.exit(255)
+            else:
+                env.Append(LINKFLAGS=["-fuse-ld=mold"])
         else:
-            print("Using LLD with GCC is not supported yet. Try compiling with 'use_llvm=yes'.")
+            env.Append(LINKFLAGS=["-fuse-ld=%s" % env["linker"]])
+
+    if env["use_thinlto"]:
+        if not env["use_llvm"] or env["linker"] != "lld":
+            print("ThinLTO is only compatible with LLVM and the LLD linker, use `use_llvm=yes linker=lld`.")
             sys.exit(255)
+        else:
+            env["use_lto"] = True  # ThinLTO implies LTO
 
     if env["use_coverage"]:
         env.Append(CCFLAGS=["-ftest-coverage", "-fprofile-arcs"])
@@ -164,16 +182,15 @@ def configure(env):
             env.Append(LINKFLAGS=["-fsanitize=memory"])
 
     if env["use_lto"]:
-        if not env["use_llvm"] and env.GetOption("num_jobs") > 1:
+        if env["use_thinlto"]:
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        elif not env["use_llvm"] and env.GetOption("num_jobs") > 1:
             env.Append(CCFLAGS=["-flto"])
             env.Append(LINKFLAGS=["-flto=" + str(env.GetOption("num_jobs"))])
         else:
-            if env["use_lld"] and env["use_thinlto"]:
-                env.Append(CCFLAGS=["-flto=thin"])
-                env.Append(LINKFLAGS=["-flto=thin"])
-            else:
-                env.Append(CCFLAGS=["-flto"])
-                env.Append(LINKFLAGS=["-flto"])
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
 
         if not env["use_llvm"]:
             env["RANLIB"] = "gcc-ranlib"