[IA64] fix stack alignment for ia32 signal handlers

This fixes the setup of the alignment of the signal frame, so that all
signal handlers are run with a properly aligned stack frame.

The current code "over-aligns" the stack pointer so that the stack frame
is effectively always mis-aligned by 4 bytes.  But what we really want
is that on function entry ((sp + 4) & 15) == 0, which matches what would
happen if the stack were aligned before a "call" instruction.

i386 and x86_64 are already fixed by d347f372



Signed-off-by: Markus F.X.J. Oberhumer <markus@oberhumer.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>