Fix gcc libunwind build.

r270692 seems to have broken gcc builds of libunwind. This is because
statements like:
  static_assert(check_fit<Registers_or1k, unw_context_t>::does_fit,
                "or1k registers do not fit into unw_context_t");
Do not work when static_assert is a macro taking two parameters, the
extra comma separating the template parameters confuses the pre-processor.
The fix is to change those statements to:
  static_assert((check_fit<Registers_or1k, unw_context_t>::does_fit),
                "or1k registers do not fit into unw_context_t");

Also fixed a gcc warning about a trivial un-intended narrowing.

Differential revision: http://reviews.llvm.org/D20119

llvm-svn: 270925