Include the fs_base and gs_base registers in amd64 target descriptions.

This permits these registers to be used with non-Linux targets.

gdb/ChangeLog:

	* features/Makefile (amd64.dat, amd64-avx.dat, amd64-mpx.dat)
	(amd64-avx-mpx.dat, amd64-avx-avx512.dat)
	(amd64-avx-mpx-avx512-pku.dat): Add i386/64bit-segments.xml in
	those rules.
	* features/i386/amd64-avx-avx512.xml: Add 64bit-segments.xml.
        * features/i386/amd64-avx-mpx-avx512-pku.xml: Add 64bit-segments.xml.
	* features/i386/amd64-avx-mpx.xml: Add 64bit-segments.xml.
	* features/i386/amd64-avx.xml: Add 64bit-segments.xml.
	* features/i386/amd64-mpx.xml: Add 64bit-segments.xml.
	* features/i386/amd64.xml: Add 64bit-segments.xml.
	* features/i386/amd64-avx-avx512.c: Regenerated.
	* features/i386/amd64-avx-mpx-avx512-pku.c: Regenerated.
	* features/i386/amd64-avx-mpx.c: Regenerated.
	* features/i386/amd64-avx.c: Regenerated.
	* features/i386/amd64-mpx.c: Regenerated.
	* features/i386/amd64.c: Regenerated.
	* regformats/i386/amd64-avx-avx512.dat: Regenerated.
	* regformats/i386/amd64-avx-mpx-avx512-pku.dat: Regenerated.
	* regformats/i386/amd64-avx-mpx.dat: Regenerated.
	* regformats/i386/amd64-avx.dat: Regenerated.
	* regformats/i386/amd64-mpx.dat: Regenerated.
	* regformats/i386/amd64.dat: Regenerated.