PowerPC, don't use bfd reloc howto in md_assemble

We support source like the following
 .data
 .quad x-.
 .space 8
x:
where at the time the .quad line is assembled, x is unknown so a fixup
is emitted for later evaluation.  This is supported for data even when
the target may not have relocations for the expression, for example,
32-bit powerpc targets lack a 64-bit reloc.  As long as the fixup
resolves at assembly time, gas is happy.

The idea of this patch is to support fixups that resolve at assembly
time for instructions too, even when the target might lack the
necessary relocations (and thus no howto).

	* config/tc-ppc.c (fixup_size): New function.
	(md_assemble): Use it to derive size and pcrel directly
	from fixup reloc type.