build: Install QEMU

Although most software can be cross-compiled, there are quite a few edge
cases where executables built for the target system need to be run on
the build host.  Notably, `ldconfig` only considers libraries for the
same machine type as the tool itself, so it's impossible to run it in a
cross-compiled root.  In order to get passed these situations, we can
use QEMU in user mode to emulate the target architecture, allowing ARM
executables to run on AMD64 machines.

lib/tools.sh

@@ -5,6 +5,7 @@ sys-apps/shadow
 EOF
 
 xargs -r emerge -vnuUj --rebuilt-binaries=y --color=y <<EOF
+app-emulation/qemu
 sec-policy/selinux-base
 sys-apps/policycoreutils
 sys-apps/kmod

portage/package.use/qemu

@@ -0,0 +1,5 @@
+# vim: set ft=gentoo-package-use :
+
+# Dustin C. Hatch <dustin@hatch.name> (11 Dec 2024)
+# Build a minimal QEMU for chrooting into the target environment
+app-emulation/qemu -* QEMU_USER_TARGETS: aarch64 PYTHON_TARGETS: python3_12