Add feature reference in .dat files

Add feature reference in .dat files

For all targets which use the newer style target descriptions, add a
"feature" marker in the dat files.
Update regdat.sh to parse feature, but do not use it (yet).

gdb/
	* features/Makefile: Add feature marker to targets with new style
	target descriptions.
	* regformats/aarch64.dat: Regenerate.
	* regformats/i386/amd64-avx-avx512-linux.dat: Likewise.
	* regformats/i386/amd64-avx-linux.dat: Likewise.
	* regformats/i386/amd64-avx-mpx-avx512-pku-linux.dat: Likewise.
	* regformats/i386/amd64-avx-mpx-linux.dat: Likewise.
	* regformats/i386/amd64-linux.dat: Likewise.
	* regformats/i386/amd64-mpx-linux.dat: Likewise.
	* regformats/i386/amd64.dat: Likewise.
	* regformats/i386/i386-avx-avx512-linux.dat: Likewise.
	* regformats/i386/i386-avx-linux.dat: Likewise.
	* regformats/i386/i386-avx-mpx-avx512-pku-linux.dat: Likewise.
	* regformats/i386/i386-avx-mpx-linux.dat: Likewise.
	* regformats/i386/i386-linux.dat: Likewise.
	* regformats/i386/i386-mmx-linux.dat: Likewise.
	* regformats/i386/i386-mpx-linux.dat: Likewise.
	* regformats/i386/i386.dat: Likewise.
	* regformats/i386/x32-avx-avx512-linux.dat: Likewise.
	* regformats/i386/x32-avx-linux.dat: Likewise.
	* regformats/i386/x32-linux.dat: Likewise.
	* regformats/tic6x-c62x-linux.dat: Likewise.
	* regformats/tic6x-c64x-linux.dat: Likewise.
	* regformats/tic6x-c64xp-linux.dat: Likewise.
	* regformats/regdat.sh: Parse feature marker.