libgcc, rs6000: Fix bump size for powerpc64 elfv1 ABI [PR108727]

As PR108727 shows, when cleanup code called by the stack
unwinder calls function _Unwind_Resume, it goes via plt
stub like:

   function 00000000.plt_call._Unwind_Resume:

=> 0x0000000010003580 <+0>:     std     r2,40(r1)
   0x0000000010003584 <+4>:     ld      r12,-31760(r2)
   0x0000000010003588 <+8>:     mtctr   r12
   0x000000001000358c <+12>:    ld      r2,-31752(r2)
   0x0000000010003590 <+16>:    cmpldi  r2,0
   0x0000000010003594 <+20>:    bnectr+
   0x0000000010003598 <+24>:    b       0x100031a4
                                        <_Unwind_Resume@plt>

It wants to save TOC base (r2) to r1 + 40, but we only
bump the stack segment by 32 bytes as follows:

   stdu %r29,-32(%r3)

It means the access is out of the stack segment allocated
by __generic_morestack, once the touch area isn't writable
like this failure shows, it would cause segment fault.

So fix the bump size with one reasonable value PARAMS.

	PR libgcc/108727

libgcc/ChangeLog:

	* config/rs6000/morestack.S (__morestack): Use PARAMS for new stack
	bump size.

libgcc/config/rs6000/morestack.S

@@ -205,12 +205,12 @@ ENTRY0(__morestack)
 	bl JUMP_TARGET(__generic_morestack)
 
 # Start using new stack
-	stdu %r29,-32(%r3)		# back-chain
+	stdu %r29,-PARAMS(%r3)		# back-chain
 	mr %r1,%r3
 
 # Set __private_ss stack guard for the new stack.
 	ld %r12,NEWSTACKSIZE_SAVE(%r29)	# modified size
-	addi %r3,%r3,BACKOFF-32
+	addi %r3,%r3,BACKOFF-PARAMS
 	sub %r3,%r3,%r12
 # Note that a signal frame has $pc pointing at the instruction
 # where the signal occurred.  For something like a timer