git: openjdk/jdk-sandbox: shen-late-barriers: Add .m4 file

[duke]

Changeset: bfe6bb5b
Branch: shen-late-barriers
Author:    Cesar Soares <dcsl at amazon.com>
Date:      2026-01-21 23:33:43 +0000
URL:       https://git.openjdk.org/jdk-sandbox/commit/bfe6bb5b5f21bc3dc880cba306dec1b8485ddcee

Add .m4 file

! .gitignore
! src/hotspot/cpu/aarch64/gc/shenandoah/shenandoah_aarch64.ad
+ src/hotspot/cpu/aarch64/gc/shenandoah/shenandoah_aarch64.m4