resources: Update M5_ISA for aarch64 and arm builds of src/simple.

The m5 utility build target for aarch64 was changed to `arm64` in [1].
Update the Makefile for the src/simple Resources to use this new
target to build the utility.

[1] https://gem5-review.googlesource.com/c/public/gem5/+/27757

Change-Id: I142382b7c211c6d17b2b4dbb4abe26aa3947adbd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/57929
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/src/simple/Makefile b/src/simple/Makefile
index 2b39334..94c0796 100644
--- a/src/simple/Makefile
+++ b/src/simple/Makefile
@@ -105,9 +105,9 @@
 M5_ISA = x86
 PREFIX =
 else
-M5_ISA = $(ISA)
 ifeq ($(ISA),aarch64)
 CCFLAGS_ISA = -Xassembler -march=all
+M5_ISA = arm64
 # Input files just for this ISA.
 ISA_BARE_INS = semihost_exit
 # Exclude baremetal only builds.
@@ -117,6 +117,7 @@
 #ISA_USER_INS =
 PREFIX = aarch64-linux-gnu-
 else
+M5_ISA = $(ISA)
 ifeq ($(ISA),arm)
 PREFIX = arm-linux-gnueabihf-
 else
@@ -205,4 +206,3 @@
 
 mkdir-user:
 	mkdir -p '$(OUT_USER_DIR)/$(ISA)'
-