loader.efi: Fix some arm64 PE metadata

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