rs6000, Add missing overloaded bcd builtin tests, documentation

rs6000, Add missing overloaded bcd builtin tests, documentation

Currently we have the documentation for __builtin_vec_bcdsub_{eq,gt,lt} but
not for __builtin_bcdsub_{gl}e, this patch is to supplement the descriptions
for them.  Although they are mainly for __builtin_bcdcmp{ge,le}, we already
have some testing coverage for __builtin_vec_bcdsub_{eq,gt,lt}, this patch
adds the corresponding explicit test cases as well.

gcc/ChangeLog:
	* doc/extend.texi (__builtin_bcdsub_le, __builtin_bcdsub_ge): Add
	documentation for the builti-ins.

gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/bcd-3.c (do_sub_ge, do_suble): Add functions
	to test builtins __builtin_bcdsub_ge and __builtin_bcdsub_le.