[compiler-rt][AVR] Use correct return value for __ledf2 etc

Previously the default was long, which is 32-bit on AVR. But avr-gcc
expects a smaller value: it reads the return value from r24.

This is actually a regression from https://reviews.llvm.org/D98205.
Before D98205, the return value was an enum (which was 2 bytes in size)
which was compatible with the 1-byte return value that avr-gcc was
expecting. But long is 4 bytes and thus places the significant return
value in a different register.

Differential Revision: https://reviews.llvm.org/D124939