The callee-saved registers must be saved because for the c-shared case
this code is invoked from C code in the system library, and that code
expects the registers to be saved.  The tests were passing because in
the normal case the code calls a cgo function that naturally saves
callee-saved registers anyhow.  However, it fails when the code takes
the non-cgo path.

Change-Id: I9c1f5e884f5a72db9614478049b1863641c8b2b9
Reviewed-on: https://go-review.googlesource.com/9114Reviewed-by: David Crawshaw <crawshaw@golang.org>