posix: Correctly block/unblock all signals on Linux posix_spawn
This patch correctly block and unblocks all signals when executing
Linux posix_spawn by using the __libc_signal_{un}block_all functions
instead of default sigprocmask. The latter might remove both
SIGCANCEL and SIGSETXID from the blocked signal list.
Checked on x86_64, i686, powerpc64le, and aarch64.
* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Correctly block and unblock
all signals when executing the clone vfork child.
(SIGALL_SET): Remove macro.