Restore qemu performance fix for Ubuntu Noble (#9371)

This restores a performance boost introduced via #6644, which appears to have been accidentally reverted in 021af5d. Additionally, a note was added to the config to indicate why that setting is important.

A write up for this issue was posted to the Armbian forums at https://forum.armbian.com/topic/57887-arm64-performance-regression-in-qemu/.

Author: tparys

config/sources/arm64.conf

@@ -37,4 +37,8 @@ else
 	display_alert "arch code" "ATF_COMPILE is 'no'" "debug"
 fi
 
+# IMPORTANT - Prevents ~5x slowdown caused by CVE-2023-4039 patchset, when
+# emulating ARM64 software via qemu. Has no effect when run on native ARM64.
+declare -gx QEMU_CPU="cortex-a53"
+
 true # make sure to exit with 0 status; this protects against shortcircuits etc above.