mesa: Replace a priori knowledge of gcc builtins with configure tests.
Presumbly this will let clang and other compilers use the built-ins as well. Notice two changes specifically: - in _mesa_next_pow_two_64(), always use __builtin_clzll and add a static assertion that this is safe. - in macros.h, remove the clang-specific definition since it should be able to detect __builtin_unreachable in configure. Reviewed-by: Jason Ekstrand <firstname.lastname@example.org> [C bits] Reviewed-by: Ian Romanick <email@example.com>
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