PowerPC: Set long double size for IBM/IEEE.

PowerPC: Set long double size for IBM/IEEE.

As I was working with compilers where the long double default was 64-bit, it
became annoying to have to use two options to switch to one of the 128-bit long
double types (i.e. you need both -mlong-double-128 and the
-mabi={ieee,ibm}longdouble to switch the long double type).

I did this patch so that if you explicitly set the long double ABI via the
-mabi= option, it would automatically set the long double size if that was not
set explicitly.

gcc/
2021-01-13  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_option_override_internal): If the
	user explicitly used -mabi=ieeelongdouble or -mabi=ibmlongdouble,
	set the long double size to 128.
	* doc/invoke.texi (PowerPC options): Document that an explicit
	-mabi=ieeelongdouble or -mabi=ibmlongdouble implicitly sets
	-mlong-double-128.