Compile issue with Neon optimizations on ARM Fedora GCC 15

See build log here https://bugzilla.mozilla.org/show_bug.cgi?id=2041214 On Firefox we have worked around the issue by disabling Neon for gcc for now.

Claude thinks this is the cause: The error is GCC-15 strictness about NEON intrinsic signedness: vabdq_s8 returns int8x16_t, but vpaddlq_u8 wants uint8x16_t. Older GCC accepted the implicit conversion; GCC 15 doesn't.