arm64: Mark EFI memory as userspace non-executable

FEAT_PAN3 adds the SCTLR_EL1.EPAN field. This tells the hardware to
raise a permission abort when userspace could execute a page, i.e. the
ATTR_S1_UXN field is clear in the page table.

This causes issues for the EFI runtime as we only mark non-executable
pages with this flag leading to a permission fault when accessing it.

Fix this by marking all EFI mappings with ATTR_S1_UXN. The kernel
already had this field set so was already safe.

Reported by: tuexen
Reviewed by: tuexen
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D52750