[Linux] Hack around Linux/sparc <bits/stdio-ldbl.h>

I've been using this hack to work around the Linux/sparc64 compile failure
described in Issue #47994
<https://github.com/llvm/llvm-project/issues/47994>, especially since the
underlying glibc PR build/27558
<https://sourceware.org/bugzilla/show_bug.cgi?id=27558> doesn't seem to be
making progress and some fix is required to have LLVM build on
`sparc64-unknown-linux-gnu` at all, as evidenced on the buildbot.

Tested on `sparc64-unknown-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D133405