posix_spawn: use a larger min stack for -fstack-check [BZ #21253]

When glibc is built with -fstack-check, trying to use posix_spawn can
lead to segfaults due to gcc internally probing stack memory too far.
The new spawn API will allocate a minimum of 1 page, but the stack
checking logic might probe a couple of pages.  When it tries to walk
them, everything falls apart.

The gcc internal docs [1] state the default interval checking is one
page.  Which means we need two pages (the current one, and the next
probed).  No target currently defines it larger.

Further, it mentions that the default minimum stack size needed to
recover from an overflow is 4/8KiB for sjlj or 8/12KiB for others.
But some Linux targets (like mips and ppc) go up to 16KiB (and some
non-Linux targets go up to 24KiB).

Let's create each child with a minimum of 32KiB slack space to support
them all, and give us future breathing room.

No test is added as existing ones crash.  Even a simple call is
enough to trigger the problem:
	char *argv[] = { "/bin/ls", NULL };
	posix_spawn(NULL, "/bin/ls", NULL, NULL, argv, NULL);

[1] https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gccint/Stack-Checking.html

(cherry picked from commit 21f042c8)