readme: factor out hardcoded paths on code snippets with getvar

run

@@ -193,11 +193,6 @@ fi
 
 if "$common_gem5"; then
   memory="${memory}B"
-  if [ "$common_arch" = x86_64 ]; then
-    gem5_arch=X86
-  else
-    gem5_arch=ARM
-  fi
   if "$trace_enabled"; then
     gem5opts="${gem5opts} --debug-flags='${trace_type}' \\
 "
@@ -205,7 +200,7 @@ if "$common_gem5"; then
   gem5_common="\
 M5_PATH='${common_gem5_system_dir}' \\
 ${debug_vm}\
-'${common_gem5_build_dir}/${gem5_arch}/gem5.${common_gem5_build_type}' \\
+'${common_exec}' \\
 --debug-file=trace.txt \\
 ${gem5opts}\
 -d '${common_m5out_dir}' \\
@@ -276,14 +271,14 @@ else
   fi
   if "$prebuilt"; then
     common_mkdir
-    qemu_executable="qemu-system-${common_arch}"
+    qemu_exec="qemu-system-${common_arch}"
   else
-    qemu_executable="${common_qemu_variant_dir}/${common_arch}-softmmu/qemu-system-${common_arch}"
+    qemu_exec="${common_qemu_exec}"
   fi
   extra_flags="${extra_flags_qemu}${extra_flags}"
   qemu_common="\
 ${debug_vm}\
-${qemu_executable} \\
+${qemu_exec} \\
 -device rtl8139,netdev=net0 \\
 -gdb 'tcp::${common_gdb_port}' \\
 -kernel '${common_linux_image}' \\