arch,cpu: Rename arch/registers.hh to arch/vecregs.hh.

The only thing still in arch/registers.hh were related to vector
registers. To make it obvious that nothing else should be added, this
change renames the file so that it has the much less generic name
arch/vecregs.hh.

Change-Id: I729697dc576e1978047688d9700dc07ff9b17044
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42686
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -61,9 +61,9 @@ env.SwitchingHeaders(
         isa.hh
         locked_mem.hh
         page_size.hh
-        registers.hh
         remote_gdb.hh
         types.hh
+        vecregs.hh
         '''),
     env.subst('${TARGET_ISA}'))