This change is the Zig counterpart to https://reviews.llvm.org/D110413
Since we lower some libcalls directly (just like clang does), we need to
make sure that the ABI we call with matches the ABI of the compiler-rt
we are providing (and also the ABI expected by LLVM).
While I was at it, I noticed some flawed vector handling in the binary
soft float ops in stage 1, so I shored up the logic a bit and expanded
an existing test to cover the missing functionality.
d182e2ebda