[sanitizer] Fix __sanitizer_kernel_sigset_t endianness issue

setuid(0) hangs on SystemZ under TSan because TSan's BackgroundThread
ignores SIGSETXID. This in turn happens because internal_sigdelset()
messes up the mask bits on big-endian system due to how
__sanitizer_kernel_sigset_t is defined.

Commit d9a1a53b ("[ESan] [MIPS] Fix workingset-signal-posix.cpp on
MIPS") fixed this for MIPS by adjusting the __sanitizer_kernel_sigset_t
definition. Generalize this by defining __SANITIZER_KERNEL_NSIG based
on kernel's _NSIG and using uptr[] for __sanitizer_kernel_sigset_t.sig
on all platforms.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D105629