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.

.github/workflows/linux.yml

@@ -38,7 +38,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-14-linux-v1
+          key: llvm-source-14-linux-v2
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -179,7 +179,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-14-linux-asserts-v1
+          key: llvm-source-14-linux-asserts-v2
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -276,7 +276,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-14-linux-v1
+          key: llvm-source-14-linux-v2
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include

.github/workflows/windows.yml

@@ -37,7 +37,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-14-windows-v1
+          key: llvm-source-14-windows-v2
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include