cmd/go: correct gccgo buildid file on ARM

Bring in https://golang.org/cl/135297 from the gc repository to fix a
GCC bug report.

Original CL description:

    The GNU assembler for ARM treats @ as a comment character, so section
    types must be written using % instead.

    Fixes https://gcc.gnu.org/PR87260.

Change-Id: Ia53be178254f4e28d7e2c51031f5deb12448a431
Reviewed-on: https://go-review.googlesource.com/135360
Reviewed-by: Cherry Zhang <cherryyz@google.com>