[clang, test] Fix Clang :: Headers/max_align.c on 64-bit SPARC

Clang :: Headers/max_align.c currently FAILs on 64-bit SPARC:

  error: 'error' diagnostics seen but not expected: 
    File /vol/llvm/src/clang/dist/test/Headers/max_align.c Line 12: static_assert failed due to requirement '8 == _Alignof(max_align_t)' ""
  1 error generated.

This happens because SuitableAlign isn't defined for SPARCv9 unlike SPARCv8
(which uses the default of 64 bits).  gcc's sparc/sparc.h has

  #define BIGGEST_ALIGNMENT (TARGET_ARCH64 ? 128 : 64)

This patch sets SuitableAlign to match and updates the corresponding testcase.

Tested on sparcv9-sun-solaris2.11.

Differential Revision: https://reviews.llvm.org/D64487

llvm-svn: 366820