ppc: Add configure test and macros to replace register names.

ppc: Add configure test and macros to replace register names.

* aclocal.m4 (GMP_ASM_POWERPC_R_REGISTERS): New configure test,
adapted from corresponding test in GMP's acinlude.m4.
* configure.ac (ASM_PPC_WANT_R_REGISTERS): New substituted
variable. Set using GMP_ASM_POWERPC_R_REGISTERS, when powerpc64
assembly code is enabled.
* config.m4.in: Substituted here.
* powerpc64/machine.m4: Check ASM_PPC_WANT_R_REGISTERS, and
if needed, replace register names like r0, r1, ... with integers.
* Makefile.in (%.asm): Include m4-utils.m4 for preprocessing of .asm
files, and include config.m4 before machine.m4.