[dev.link] cmd/link: set direct fn address in dwarf gen where possible

If we are internal linking a static executable, and address assignment
has happened, then when emitting some parts of DWARF we can just emit
a function address directly instead of generating a relocation. For
external linking or other build modes, we are generating a relocatable
binary so we still need to emit relocations.

This CL inspired by Cherry's similar CL for pclntab at
https://go-review.googlesource.com/c/go/+/228478.

Change-Id: Ib03fbe2dd72d0ba746bf46015e0f2d6c3f3d53ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/228537
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/loader/symbolbuilder.go

@@ -405,3 +405,24 @@ func (sb *SymbolBuilder) AddSize(arch *sys.Arch, tgt Sym) int64 {
 	sb.setReachable()
 	return sb.addSymRef(tgt, 0, objabi.R_SIZE, arch.PtrSize)
 }
+
+// GenAddAddrPlusFunc returns a function to be called when capturing
+// a function symbol's address. In later stages of the link (when
+// address assignment is done) when doing internal linking and
+// targeting an executable, we can just emit the address of a function
+// directly instead of generating a relocation. Clients can call
+// this function (setting 'internalExec' based on build mode and target)
+// and then invoke the returned function in roughly the same way that
+// loader.*SymbolBuilder.AddAddrPlus would be used.
+func GenAddAddrPlusFunc(internalExec bool) func(s *SymbolBuilder, arch *sys.Arch, tgt Sym, add int64) int64 {
+	if internalExec {
+		return func(s *SymbolBuilder, arch *sys.Arch, tgt Sym, add int64) int64 {
+			if v := s.l.SymValue(tgt); v != 0 {
+				return s.AddUint(arch, uint64(v+add))
+			}
+			return s.AddAddrPlus(arch, tgt, add)
+		}
+	} else {
+		return (*SymbolBuilder).AddAddrPlus
+	}
+}