rs6000: Fix restored rs6000_long_double_type_size

rs6000: Fix restored rs6000_long_double_type_size

As mentioned in the "Fallout: save/restore target options in handle_optimize_attribute"
thread, we need to support target option restore
of rs6000_long_double_type_size == FLOAT_PRECISION_TFmode.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_option_override_internal): When
	a target option is restored, it can have
	rs6000_long_double_type_size set to FLOAT_PRECISION_TFmode
	and error should not be emitted.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pragma-optimize.c: New test.