arch_init: Fix AltiVec build on Darwin/ppc

arch_init: Fix AltiVec build on Darwin/ppc

Commit f29a56147b66845914d0a645bf9b4c5bb9a6af57 (implement
-no-user-config command-line option (v3)) introduced uses of bool
in arch_init.c. Shortly before that usage is support code for
AltiVec (conditional to __ALTIVEC__).

GCC's altivec.h may in a !__APPLE_ALTIVEC__ code path redefine bool,
leading to type mismatches. altivec.h recommends to #undef for C++
compatibility, but doing so in C leads to bool remaining undefined.

Fix by redefining bool to _Bool as mandated for stdbool.h by POSIX.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>