build: treat aarch64 as arm64

`Makefile` is now consistent with `configure` regarding
how we treat aarch64.

PR-URL: https://github.com/nodejs/node/pull/5191
Fixes: https://github.com/nodejs/node/issues/5175
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Makefile

@@ -272,6 +272,9 @@ else
 ifeq ($(DESTCPU),arm)
 ARCH=arm
 else
+ifeq ($(DESTCPU),aarch64)
+ARCH=arm64
+else
 ifeq ($(DESTCPU),ppc64)
 ARCH=ppc64
 else
@@ -283,6 +286,7 @@ endif
 endif
 endif
 endif
+endif
 
 # enforce "x86" over "ia32" as the generally accepted way of referring to 32-bit intel
 ifeq ($(ARCH),ia32)