support: Add support_stack_alloc

The code to allocate a stack from xsigstack is refactored so it can
be more generic.  The new support_stack_alloc() also set PROT_EXEC
if DEFAULT_STACK_PERMS has PF_X.  This is required on some
 architectures (hppa for instance) and trying to access the rtld
global from testsuite will require more intrusive refactoring
in the ldsodefs.h header.

Checked on x86_64-linux-gnu and i686-linux-gnu.  I also ran
tst-xsigstack on both hppa and ia64.