ART: Remove some of the Mac craziness

We rely on new-enough Clang/LLVM builds nowadays. The integrated
assembler supports named parameters. Throw away most of the
old duplication (effectively cutting support for older Clang
versions). The only required duplications are:

1) Clang as does not support .altmacro. However, the Clang
preprocessor works different wrt/ the GCC preprocessor
and does not give us trouble with inserted spaces.

2) On the Mac, symbols are prefixed with an underscore.

This should help to avoid breaking the Mac build when changing
the assembly code, and prepare for a complete Clang-only build
for x86 and x86-64. Switching to the integrated assembler for
the host build may be done in a follow-up CL.

Bug: 17443165
Change-Id: I1a077d4b612abc2b1b851c1bdabb5008a52e5aa6