runtime/cgo: fix build

Two forgotten renames from last CL.

TBR=r
CC=golang-dev
https://golang.org/cl/5502046

diff --git a/src/pkg/runtime/cgo/386.S b/src/pkg/runtime/cgo/gcc_386.S
similarity index 100%
rename from src/pkg/runtime/cgo/386.S
rename to src/pkg/runtime/cgo/gcc_386.S

diff --git a/src/pkg/runtime/cgo/arm.S b/src/pkg/runtime/cgo/gcc_arm.S
similarity index 100%
rename from src/pkg/runtime/cgo/arm.S
rename to src/pkg/runtime/cgo/gcc_arm.S