The difference is that GCC doesn't define __ppc__ or __ppc64__ (but rather __powerpc__ and __powerpc64__) so it doesn't try to enable the altivec code... The pfft BUILD.gn file explicitly sets PFFFT_SIMD_DISABLE on a bunch of platforms (including ppc64), and moz.build doesn't bother to do that.
Bug 1738872 Comment 20 Edit History
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The difference is that GCC doesn't define `__ppc__` or `__ppc64__` (but rather `__powerpc__` and `__powerpc64__`) so it doesn't try to enable the altivec code... The pfft BUILD.gn file explicitly sets `PFFFT_SIMD_DISABLE` on a bunch of platforms (including ppc64), and moz.build doesn't bother to do that.