Revert "compiler: make genSym use C-based atomic increment on non-JS 32-bit platforms"

This reverts commit 615eb855. It was
originally intended to fix #24449, but it was merely sweeping the bug
under the rug. 3836a110 has properly
fixed the fragile test, and we no longer need the C version of genSym.
Furthermore, the C implementation causes trouble when compiling with
clang that targets i386 due to alignment warning and libatomic linking
issue, so it makes sense to revert it.