cmd/internal/ld: add -buildmode=c-shared as an alternative to -shared

The linker currently (on some platforms) takes a -shared flag, which means
approximately what -buildmode=c-shared means in the in the proposed "Go
Execution Modes" document. As part of implementing other modes, the term
"shared" becomes horribly overloaded, so this replaces -shared with a
-buildmode argument instead (which currently only handles -buildmode=c-shared
and the default -buildmode=exe -- no new behaviour here).

As the linker support for -shared was in 1.4 this retains it as an alias.

Change-Id: Id2ebb8e05ee07f46208a554bc2622d0e67b47082
Reviewed-on: https://go-review.googlesource.com/8304
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/ld/elf.go

@@ -1658,7 +1658,7 @@ func doelf() {
 		Addstring(shstrtab, ".note.GNU-stack")
 	}
 
-	if Flag_shared != 0 {
+	if Buildmode == BuildmodeCShared {
 		Addstring(shstrtab, ".init_array")
 		switch Thearch.Thechar {
 		case '6', '7', '9':