Investigate why `-neon,-fp-armv8` segfaults building `compiler_builtins`

[ojeda]

[ojeda]

This is a very old issue from one of the first PRs -- closing!

We moved at some point to -Ctarget-feature="-neon", and nowadays we are, in fact, just moving to a softfloat target to avoid that, and possibly in the future a nofloat one (or an equivalent to -mgeneral-regs-only).