support `reg_byte` registers #2518

Job	Run time
build (gcc-15.deb, --std-tests)	7m 16s
build (gcc-15.deb, --test-libcore)	15m 0s
build (gcc-15.deb, --extended-rand-tests)	8m 30s
build (gcc-15.deb, --extended-regex-example-tests)	7m 6s
build (gcc-15.deb, --extended-regex-tests)	11m 36s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 0s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 52s
build (gcc-15.deb, --projects)	11m 31s
build (gcc-15-without-int128.deb, --std-tests)	5m 49s
build (gcc-15-without-int128.deb, --test-libcore)	14m 25s
build_system	29s
build (gcc-15-without-int128.deb, --extended-rand-tests)	8m 51s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	6m 54s
build (gcc-15-without-int128.deb, --extended-regex-tests)	12m 22s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 54s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 38s
duplicates	2s
build (gcc-15-without-int128.deb, --projects)	11m 52s
success	1s
	2h 50m 8s

Provide feedback