util: Move m5op_arm_A64.S into an aarch64 subdirectory.

Also rename it to m5op.S. These files will be distinguished by what
directory they're in, rather than by their name.

Change-Id: I3a89d529bb5b760579df22234cf9bf870711b56e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27216
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/m5/src/SConscript

@@ -29,7 +29,7 @@ Import('*')
 
 # Raw source files.
 m5_mmap = 'm5_mmap.c'
-m5op = 'm5op_arm_A64.S'
+m5op = 'aarch64/m5op.S'
 m5 = 'm5.c'
 jni = 'jni_gem5Op.c'
 lua = 'lua_gem5Op.c'