sys_stat-h tests: Fix a static_assert failure on glibc/hppa. Reported by Santiago Vila <sanvila@debian.org> in: <https://lists.gnu.org/archive/html/bug-m4/2025-09/msg00000.html>. * tests/test-sys_stat-h.c [__GLIBC__ && __hppa]: Don't check that blksize_t and blkcnt_t are signed. * doc/posix-headers/sys_stat.texi: Mention that these types are unsigned on this platform.