scr_exc_alert, the process-wide exception alert of worker executables on
targets without the ELF per-thread pending word, was a tentative
definition. That branch compiles only for Mach-O, COFF and WASI, so the
macOS runtime build emitted it as a COMMON symbol, which library
localization keeps global, and the COMMON-symbol test failed on macOS
while passing on Linux.
Initialize it explicitly, and have the test also compile every runtime
source for aarch64-macos through zig cc when zig is on PATH and the host
is not macOS, so declarations in __APPLE__ branches are checked on Linux
too.
Chris Tate
03e78caffc