This change adds support for compiler-rt, which supports float64 (unlike
libgcc for AVR). This gets a number of tests to pass that require
float64 support.
We're still using libgcc with this change, but libgcc will probably be
removed eventually once AVR support in compiler-rt is a bit more mature.
I've also pushed a fix for a small regression in our
xtensa_release_14.0.0-patched LLVM branch that has also been merged
upstream. Without it, a floating point comparison against zero always
returns true which is certainly a bug. It is necessary to correctly
print floating point values.