cmd/link: set S_ATTR_PURE_INSTRUCTIONS bit for text section on darwin

Mac otool and llvm-objdump distinguishes a Mach-O section is
text or data by looking at S_ATTR_PURE_INSTRUCTIONS bit. Without
this bit it thinks our function symbols are data, not functions.
Set this bit for text section to make otool/objdump happy.

Fixes #24706.

Change-Id: I5236482cb9a72474c23fbea0f35d5b5cc8491ea4
Reviewed-on: https://go-review.googlesource.com/105256
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/macho.go

@@ -438,6 +438,10 @@ func machoshbits(ctxt *Link, mseg *MachoSeg, sect *sym.Section, segname string)
 		msect.flag |= S_ATTR_SOME_INSTRUCTIONS
 	}
 
+	if sect.Name == ".text" {
+		msect.flag |= S_ATTR_PURE_INSTRUCTIONS
+	}
+
 	if sect.Name == ".plt" {
 		msect.name = "__symbol_stub1"
 		msect.flag = S_ATTR_PURE_INSTRUCTIONS | S_ATTR_SOME_INSTRUCTIONS | S_SYMBOL_STUBS