[Chapel Merge] Use our spinlock to serialize gasnet poller calls

Branch: refs/heads/master
Revision: 80c6acf
Author: ronawho
Log Message:

Merge pull request #17420 from ronawho/cleanup-gn-am-poller-serialization

Use our spinlock to serialize gasnet poller calls

[reviewed by @gbtitus]

Instead of manually implementing a lock, use the spinlock wrapper. For
CHPL_ATOMICS=cstdlib the implementation is the same so this is just a
minor code cleanup. It'll have better performance for other atomic
implementations, but cstdlib is the default in most places anymore.

Modified Files:
M runtime/src/comm/gasnet/comm-gasnet.c

Compare: https://github.com/chapel-lang/chapel/compare/5430ec0f3c07...80c6acf40425