cmd/link: put runtime.framepointer_enabled in DATA instead of RODATA

On darwin, only writable symbol is exported
(cmd/link/internal/ld/macho.go:/machoShouldExport).
For plugin to work correctly, global variables, including
runtime.framepointer_enabled which is set by the linker, need
to be exported when dynamic linking. Put it in DATA so it is
exported. Also in Go it is defined as a var, which is not
read-only.

While here, do the same for runtime.goarm.

Fixes #24653.

Change-Id: I9d1b7d5a648be17103d20b97be65a901cb69f5a2
Reviewed-on: https://go-review.googlesource.com/104715
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -454,14 +454,14 @@ func (ctxt *Link) loadlib() {
 		// recording the value of GOARM.
 		if ctxt.Arch.Family == sys.ARM {
 			s := ctxt.Syms.Lookup("runtime.goarm", 0)
-			s.Type = sym.SRODATA
+			s.Type = sym.SDATA
 			s.Size = 0
 			s.AddUint8(uint8(objabi.GOARM))
 		}
 
 		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
 			s := ctxt.Syms.Lookup("runtime.framepointer_enabled", 0)
-			s.Type = sym.SRODATA
+			s.Type = sym.SDATA
 			s.Size = 0
 			s.AddUint8(1)
 		}