[builtins] Use __APPLE__ instead of __MACH__ in check
[builtins] Use __APPLE__ instead of __MACH__ in check

The latter doesn't seem to be working for all targets. This addresses
the issue introduced in r359413.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@359423 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt