Use uname -m as -p doesn't always work

See https://unix.stackexchange.com/questions/307955/uname-p-i-are-unknown
`uname -m` returns machine type, which is enough for this script, `uname -p` doesn't work on most Linux distros.

installer/install-marksman.sh

@@ -3,7 +3,7 @@
 set -e
 
 os=$(uname -s | tr "[:upper:]" "[:lower:]")
-if [ $(uname -p) = "x86_64" ];
+if [ $(uname -m) = "x86_64" ];
 then
   arch="x64"
 else