Make code more compatible with gcc. Change
__atomic_store
__atomic_load
__atomic_exchange
__atomic_compare_exchange
Also add (include/stdatomic.h, lib/stdatomic.c):
atomic_thread_fence
atomic_signal_fence
atomic_is_lock_free
And gcc extensions (tcctok.h, tccgen.c, lib/stdatomic.c):
__atomic_fetch_nand
__atomic_and_fetch
__atomic_sub_fetch
__atomic_or_fetch
__atomic_xor_fetch
__atomic_and_fetch
__atomic_nand_fetch
Add new file lib/atomic.S with assembly code for __atomic_compare_exchange_n
for arm/arm64/riscv. Also update lib/Makefile.
Update testcode in 124_atomic_counter and 125_atomic_misc to test new functions.
Also update tests/tests2/Makefile to run tests on arm/arm64/riscv.
