builtins: remove definition of __ARM_ARCH

__ARM_ARCH is part of the ACLE specification.  At least clang and GCC have
supported this part of the ACLE for some time now.  Let the compiler provide the
proper definition for the macro rather than try to guess it.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218095 91177308-0d34-0410-b5e6-96231b3b80d8