loader.efi: make sure kernel image is executable

The Windows Dev Kit 2023 (Volterra) has an UEFI implementation that maps
EfiLoaderData pages as non-executable. Map the kernel as EfiLoaderCode
to ensure that it can be executed.

With this change and another in review, FreeBSD boots to the mountroot
prompt if hw.pac.enable = 0 is set in loader.conf(5).

Reviewed by: andrew, imp, tsoome
Sponsored by: Berliner Linux User Group e.V.
Sponsored by: spline / FU-Berlin
Differential Revision: https://reviews.freebsd.org/D37931

(cherry picked from commit 95fa2e0aee5b7259cf4bcdea7396c4dff3241173)