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.
