dwarf2asm: Fix bootstrap on powerpc*-*-* [PR98839]

My recent dwarf2asm.c patch broke powerpc*-*-* bootstrap, while most target
define POINTER_SIZE to (cond ? cst1 : cst2) or constant, rs6000 defines
it to a variable, and the arbitrarily chosen type of that variable determines
whether we get warnings on comparison of that against signed or unsigned
ints.

Fixed by adding a cast.

2021-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/98839
	* dwarf2asm.c (dw2_assemble_integer): Cast DWARF2_ADDR_SIZE to int
	in comparison.