rs6000/test: Fix empty TU in some cases of effective targets [PR106345]

As the failure of test case gcc.target/powerpc/pr92398.p9-.c in
PR106345 shows, some test sources for some powerpc effective
targets use empty translation unit wrongly.  The test sources
could go with options like "-ansi -pedantic-errors", then those
effective target checkings will fail unexpectedly with the
error messages like:

  error: ISO C forbids an empty translation unit [-Wpedantic]

This patch is to fix empty TUs with one dummy function definition
accordingly.

	PR testsuite/106345

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_powerpc_sqrt): Add
	a function definition to avoid pedwarn about empty translation unit.
	(check_effective_target_has_arch_pwr5): Likewise.
	(check_effective_target_has_arch_pwr6): Likewise.
	(check_effective_target_has_arch_pwr7): Likewise.
	(check_effective_target_has_arch_pwr8): Likewise.
	(check_effective_target_has_arch_pwr9): Likewise.
	(check_effective_target_has_arch_pwr10): Likewise.
	(check_effective_target_has_arch_ppc64): Likewise.
	(check_effective_target_ppc_float128): Likewise.
	(check_effective_target_ppc_float128_insns): Likewise.
	(check_effective_target_powerpc_vsx): Likewise.