cmd/ld: change GC_CALL to 32-bit relative address

The current code uses 64-bit pc-relative on 64-bit systems,
but in ELF linkers there is no such thing, so we cannot
express this in a .o file. Change to 32-bit.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7383055
libmach: fix build (set and not used)

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/7401053