rs6000, Update support for vec_extract

rs6000, Update support for vec_extract

GCC maintainers:

Move the existing vector extract support in altivec.md to vsx.md
so all of the vector insert and extract support is in the same file.

The patch also updates the name of the builtins and descriptions for the
builtins in the documentation file so they match the approved builtin
names and descriptions.

The patch does not make any functional changes.

Please let me know if the changes are acceptable for mainline.  Thanks.

                  Carl Love

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gcc/ChangeLog

	    2020-08-04  Carl Love  <cel@us.ibm.com>

	* config/rs6000/altivec.md: (UNSPEC_EXTRACTL, UNSPEC_EXTRACTR)
	(vextractl<mode>, vextractr<mode>)
	(vextractl<mode>_internal, vextractr<mode>_internal for mode VI2)
	(VI2): Move to ...
	* config/rs6000/vsx.md:	(UNSPEC_EXTRACTL, UNSPEC_EXTRACTR)
	(vextractl<mode>, vextractr<mode>)
	(vextractl<mode>_internal, vextractr<mode>_internal for mode VI2)
	(VI2):  ..here.
	* doc/extend.texi: Update documentation for vec_extractl.
	Replace builtin name vec_extractr with vec_extracth.  Update
	description of vec_extracth.