(In reply to Cameron Kaiser [:spectre] from comment #12)
I'd prefer someone who can test it do that in a followup.
I can't test as part of Firefox (doesn't build, anyway) but FreeBSD bits extracted into standalone sample worked fine on ref12-ppc64.freebsd.org. NetBSD bits were tested on x86_64 after replacing
HW_NCPU (can't use powerpc* macro on non-powerpc*). OpenBSD has similar code in devel/sdl package.
In other words, if you want the perfect testing then it won't happen for years. During that time living with broken
/proc fallback doesn't seem desirable.
the "Mozilla way" seems to be runtime detection, not compile-time detection
x86_32 and x86_64 translate
__AVX__, etc. into
MOZILLA_PRESUME_* which is later used to skip runtime checks e.g.,