[compiler-rt] [builtin] Switch the return type of __atomic_compare_exchange_##n to bool

Summary:
Synchronize the function definition with the LLVM documentation.

https://llvm.org/docs/Atomics.html#libcalls-atomic

GCC also returns bool for the same atomic builtin.

Reviewers: theraven

Reviewed By: theraven

Subscribers: theraven, dberris, jfb, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79845