cmd/go: enable -msan on freebsd/amd64

Enable -msan flag on freebsd/amd64 and amend PIE comment in
internal/work/init.go to indicate that MSAN requires PIE on all platforms
except linux/amd64.

R=go1.20

For #53298

Change-Id: I93d94efa95d7f292c23c433fb1d3f4301d820bde
Reviewed-on: https://go-review.googlesource.com/c/go/+/411275
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/go/alldocs.go

@@ -114,9 +114,9 @@
 //		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
 //	-msan
 //		enable interoperation with memory sanitizer.
-//		Supported only on linux/amd64, linux/arm64
+//		Supported only on linux/amd64, linux/arm64, freebsd/amd64
 //		and only with Clang/LLVM as the host C compiler.
-//		On linux/arm64, pie build mode will be used.
+//		PIE build mode will be used on all platforms except linux/amd64.
 //	-asan
 //		enable interoperation with address sanitizer.
 //		Supported only on linux/arm64, linux/amd64.

src/cmd/go/internal/work/build.go

@@ -71,9 +71,9 @@ and test commands:
 		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
 	-msan
 		enable interoperation with memory sanitizer.
-		Supported only on linux/amd64, linux/arm64
+		Supported only on linux/amd64, linux/arm64, freebsd/amd64
 		and only with Clang/LLVM as the host C compiler.
-		On linux/arm64, pie build mode will be used.
+		PIE build mode will be used on all platforms except linux/amd64.
 	-asan
 		enable interoperation with address sanitizer.
 		Supported only on linux/arm64, linux/amd64.

src/cmd/go/internal/work/init.go

@@ -149,9 +149,9 @@ func instrumentInit() {
 	mode := "race"
 	if cfg.BuildMSan {
 		mode = "msan"
-		// MSAN does not support non-PIE binaries on ARM64.
-		// See issue #33712 for details.
-		if cfg.Goos == "linux" && cfg.Goarch == "arm64" && cfg.BuildBuildmode == "default" {
+		// MSAN needs PIE on all platforms except linux/amd64.
+		// https://github.com/llvm/llvm-project/blob/llvmorg-13.0.1/clang/lib/Driver/SanitizerArgs.cpp#L621
+		if cfg.BuildBuildmode == "default" && (cfg.Goos != "linux" || cfg.Goarch != "amd64") {
 			cfg.BuildBuildmode = "pie"
 		}
 	}

src/internal/platform/supported.go

@@ -29,6 +29,8 @@ func MSanSupported(goos, goarch string) bool {
 	switch goos {
 	case "linux":
 		return goarch == "amd64" || goarch == "arm64"
+	case "freebsd":
+		return goarch == "amd64"
 	default:
 		return false
 	}