util: unbreak non-linux mips64 build

Put linux specific path inside an ifdef. Unbreaks mips64 build on OpenBSD and likely other systems without Elf64_auxv_t.

Fixes: 88b234d7 ("gallivm: add basic mips64 support and set mcpu to mips64r5 on ls3a4000")

which was part of !11955 (merged)

There are more u_cpu_detect.c build fixes in !6511 (merged) and !13448 (merged)