loader.efi: Fix some arm64 PE metadata

• Mark the file as an executable in the COFF header.
• Provide separate .text and .data sections.
• Provide sane file and section alignment values. These values are the defaults defined in the PE specification.
• Set appropriate characteristics for each of .text and .data.

This is required for the MS devkit to load our UEFI image.

Obtained from: OpenBSD via allanjude
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D37765