The dynamic linker on linux/386 stores the address of the vsyscall helper at a
fixed offset from the %gs register on linux/386 for easy access from PIC code.

Change-Id: I635305cfecceef2289985d62e676e16810ed6b94
Reviewed-on: https://go-review.googlesource.com/16346Reviewed-by: Ian Lance Taylor <iant@golang.org>