patch: fix build of libcxxabi with gcc

[Nick Lewycky]

The attached patch resolves a few issues building libcxxabi with gcc:

 - GCC doesn't support #pragma mark, only Apple GCC and clang. Wrap the
pragma mark's in #if 0 to make gcc ignore them but xcode still see them.
 - Wrap a bunch of "#pragma clang" lines in #ifdef __clang__.
 - Pacify gcc's -Wparenthesis in a case where it's quite reasonable.

Please review!

Nick
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