Allow disabled SIMD support on x86_64 baremetal targets

[DarkKirb]

Related to #552

Currently, on x86_64 baremetal targets and UEFI, compiling curve25519-dalek fails with the error message:

rustc-LLVM ERROR: Do not know how to split the result of this operator!

this is because these targets disable FPU support (target features -mmx,-sse,+soft-float).
It would be nice to have some sort of opt-out like sha2’s force-soft feature.

[tarcieri]

That's surprising, because cpufeatures should already take care of this: RustCrypto/utils#821