[BPF] Fix assembly parsing errors for atomic_fetch_* instructions

Fixes BPF assembler parsing errors for the following instructions:
- atomic_fetch_add
- atomic_fetch_and
- atomic_fetch_xor
- atomic_fetch_or
- cmpxchg32_32
- cmpxchg_64
- xchg32_32
- xchg_64

Also add a test to verify that all instructions could be assembled and disassembled.

Differential Revision: https://reviews.llvm.org/D147421