PowerPC: Fix rs6000-gen-builtins with build != host [PR102353]

This mimics what the main Makefile.in does: compile the generator
files under build (with Makefile.in's 'build/%.o' rule for compilation).
It also adds $(RUN_GEN) to optionally run it with valgrind and
the $(build_exeext) suffix.

Before, the .o files were compiled with $(COMPILE), causing link
error with $(LINKER_FOR_BUILD) for build != host.

gcc/
	PR target/102353
	* config/rs6000/t-rs6000 (build/rs6000-gen-builtins.o, build/rbtree.o):
	Added 'build/' to target, use build/%.o rule.
	(build/rs6000-gen-builtins$(build_exeext)): Add 'build/' and
	'$(build_exeext)' to target and 'build/' for the *.o files.
	(rs6000-builtins.c): Update for those changes; run rs6000-gen-builtins
	with $(RUN_GEN).