Fix __libc_signal_block_all on sparc64

Fix __libc_signal_block_all on sparc64

The posix_spawn on sparc issues invalid sigprocmask calls:

  rt_sigprocmask(0xffe5e15c /* SIG_??? */, ~[], 0xffe5e1dc, 8) = -1 EINVAL (Invalid argument)

Which make support/tst-support_capture_subprocess fails with random
output (due the child signal being wrongly captured by the parent).

Tracking the culprit it seems to be a wrong code generation in the
INTERNAL_SYSCALL due the automatic sigset_t used on
__libc_signal_block_all:

  return INTERNAL_SYSCALL (rt_sigprocmask, err, 4, SIG_BLOCK, &SIGALL_SET,
                          set, _NSIG / 8);

Where SIGALL_SET is defined as:

  ((__sigset_t) { .__val = {[0 ...  _SIGSET_NWORDS-1 ] =  -1 } })

Building the expanded __libc_signal_block_all on sparc64 with recent
compiler (gcc 8.3.1 and 9.1.1):

  #include <signal>

  int
  _libc_signal_block_all (sigset_t *set)
  {
    INTERNAL_SYSCALL_DECL (err);
    return INTERNAL_SYSCALL (rt_sigprocmask, err, 4, SIG_BLOCK, &SIGALL_SET,
			     set, _NSIG / 8);
  }

The first argument (SIG_BLOCK) is not correctly set on 'o0' register:

  __libc_signal_block_all:
	save    %sp, -304, %sp
	add     %fp, 1919, %o0
	mov     128, %o2
	sethi   %hi(.LC0), %o1
	call    memcpy, 0
	 or     %o1, %lo(.LC0), %o1
	add     %fp, 1919, %o1
	mov     %i0, %o2
	mov     8, %o3
	mov     103, %g1
	ta      0x6d;
	bcc,pt  %xcc, 1f
	mov     0, %g1
	sub     %g0, %o0, %o0
	mov     1, %g1
     1:	sra     %o0, 0, %i0
	return  %i7+8
	 nop

Where if SIGALL_SET is defined a const object, gcc correctly sets the
expected kernel argument in correct register:

        sethi   %hi(.LC0), %o1
        call    memcpy, 0
         or     %o1, %lo(.LC0), %o1
   ->   mov     1, %o0
	add     %fp, 1919, %o1

Another possible fix is use a static const object.  Although there
should not be a difference between a const compound literal and a static
const object, the gcc C99 status page [1] has a note stating that this
optimization is not implemented:

  "const-qualified compound literals could share storage with each
   other and with string literals, but currently don't.".

This patch fixes it by moving both sigset_t that represent the
signal sets to static const data object.  It generates slight better
code where the object reference is used directly instead of a stack
allocation plus the content materialization.

Checked on x86_64-linux-gnu, i686-linux-gnu, and sparc64-linux-gnu.

[1] https://gcc.gnu.org/c99status.html