Allow installing Alpine's mesa for x86* arches (currently breaking 'pmbootstrap qemu' for some people)

The mesa version 19.0 added to pmaports.git in https://gitlab.com/postmarketOS/pmaports/merge_requests/256 is needed for arm devices (e.g. it fixes hammerhead blackscreen). But Alpine is currently stuck on 18.2.8 because of x86_64 ATI drivers, which also need a LLVM upgrade (see this PR).

So it would be a good solution to fork the mesa aport for the arm arches, but not for x86*.

This is not supported by pmbootstrap yet:

$ pmbootstrap chroot --add=mesa
[09:47:34] NOTE: You can edit the 'arch=' line inside the APKBUILD
[09:47:34] ERROR: Can't build 'mesa' for architecture x86_64

For people where this causes breakage in qemu, see using QEMU from the host system as workaround.