Don't use clock_nanosleep on PASE (mono/mono#15988)

Don't use clock_nanosleep on PASE (mono/mono#15988)

* Don't use clock_nanosleep on PASE

PASE exports this AIX syscall so autoconf detects it, but using it
will trigger SIGILL (for unimplemented syscall), which emits a LIC
log entry type of 4700-000F.

* Change ifdef guard for clock_nanosleep in threads

It only used clock_nanosleep on Linux, but enable it for all except
PASE. Android used to be guarded against, but the reasoning was
unknown, so @akoeplinger requested to have that guard removed.

Commit migrated from https://github.com/mono/mono/commit/9ea07610ee4c2879f5d598aa6f34b6df79401047