compiler-rt: Fix "bare metal" case boolean logic
It is possible to both be bare metal and have a libc (newlib).
This libc doesn't provide very much --- not enough for CMake to think
the C toolchain works. We therefore adjust our logic so we hit the "bare
metal" case with or without libc.
The "use LLVM" bootstrap is intentionally not affected.
(cherry picked from commit bf39e322721e581f972a14e18310797260f7f35f)
(cherry picked from commit c820cd8cee30f09431d8ce78dee613360052727a)
