Posix/unpoison-alternate-stack.cpp currently XPASSes on Solaris. The XFAIL had
already been removed in D97933, but reintroduced by commit f03d29601e0951da2c88f07d4234128e14e87870 which was never posted or justified.
Given the XPASS, this obviously wasn't NFC, so I suggest to remove it again.
Tested on amd64-pc-solaris2.11 and x86_64-pc-linux-gnu.