linux: Ignore sigaction(2) flags SA_UNSUPPORTED and SA_EXPOSE_TAGBITS

SA_UNSUPPORTED was introduced in Linux 5.11 to probe support
for other flags such as SA_EXPOSE_TAGBITS, introduced
at the same time. Ignore both.

Signed-off-by: Ricardo Branco <rbranco@suse.de>
PR: 289285
Reviewed by: pouria, kib
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2163