runtime/cgo: address code review comments from iant.

https://go-review.googlesource.com/#/c/2124/2/.
misc/cgo/test/issue7989.go: require clang to build on darwin/arm.

GCC 4.2.1 does support the __sync_fetch_and_add primitve, but it
couldn't inline its implementation. libSystem on Darwin/ARM doesn't
have that function. As clang can inline that primitive, only clang
will work.

Addressing iant's comment in
https://go-review.googlesource.com/#/c/2125/2/.