use uname -m instead of arch comamnd

`arch` command is not installed at default.
see coreutils document:

> arch is not installed by default, so portable scripts should
> not rely on its existence.

https://www.gnu.org/software/coreutils/manual/html_node/arch-invocation.html

ref: google/fscrypt#92 (comment)

etc/scripts/get-stack.sh

@@ -71,9 +71,9 @@ post_install_separator() {
 
 # determines the the CPU's instruction set
 get_isa() {
-  if arch | grep -Eq 'armv[78]l?' ; then
+  if uname -m | grep -Eq 'armv[78]l?' ; then
     echo arm
-  elif arch | grep -q aarch64 ; then
+  elif uname -m | grep -q aarch64 ; then
     echo aarch64
   else
     echo x86