stand/efi: Call md_copymodules based on LP64 to fix 32-bit arm

When I refactored everything, I neglected to pass in the proper is64
value on 32-bit platforms. This corrects that. This prevented armv7 and
armv6 platforms from booting due to misaligned data in the kernel. The
only platform we support 32-bit booting in armv[67], which I apparently
neglected to test before commiting my refactoring.

Tested by: skibo
Fixes: 5d1531d9d4e7d
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37095