cmd/link: set ELF CPIC bit in mips64 objects

We already set it for mips32 objects. The native ELF linker warns when
linking PIC objects with non-PIC objects. Our objects are PIC, but we
were not marking them as such.

Fixes #20243.

Change-Id: Ifab131200b263e4c72cf81f7b131a65ac02a13a9
Reviewed-on: https://go-review.googlesource.com/42710
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -936,7 +936,7 @@ func Elfinit(ctxt *Link) {
 		fallthrough
 	case sys.AMD64, sys.ARM64, sys.MIPS64:
 		if SysArch.Family == sys.MIPS64 {
-			ehdr.flags = 0x20000000 /* MIPS 3 */
+			ehdr.flags = 0x20000004 /* MIPS 3 CPIC */
 		}
 		elf64 = true