build-all: also build gem5, document further

build-all

@@ -1,4 +1,10 @@
 #!/usr/bin/env bash
+set -eux
+# Build all archs (that we care the most about).
+# Will take forever from a clean repo, this is most useful
+# after pulling the repository to do an incremental build
+# then quickly test out all the setups.
 for arch in x86_64 arm aarch64; do
   ./build -a "$arch" -klq
+  ./build -a "$arch" -g -Gkl
 done