[compiler-rt][AVR] Fix avr_SOURCES CMake variable

D123200 did not include the generic sources, which means that only the
AVR-specific sources were compiled. With this change, generic sources
are included as expected.

Tested with the following commands:

    cmake -G Ninja -DCOMPILER_RT_DEFAULT_TARGET_TRIPLE=avr -DCOMPILER_RT_BAREMETAL_BUILD=1 -DCMAKE_C_COMPILER=clang-14 -DCMAKE_C_FLAGS="--target=avr -mmcu=avr5 -nostdlibinc -mdouble=64" ../path/to/builtins

    ninja

Differential Revision: https://reviews.llvm.org/D124969