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>