avr: use compiler-rt

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.

.circleci/config.yml

@@ -22,12 +22,12 @@ commands:
     steps:
       - restore_cache:
           keys:
-            - llvm-source-14-v1
+            - llvm-source-14-v2
       - run:
           name: "Fetch LLVM source"
           command: make llvm-source
       - save_cache:
-          key: llvm-source-14-v1
+          key: llvm-source-14-v2
           paths:
             - llvm-project/clang/lib/Headers
             - llvm-project/clang/include