loader: Move ldscripts to match more standard practices

In the larger open source community, ld scripts are foo.ldscrpt rather
than ldscript.arch like we use here. This moves the EFI ldscripts.

Sponsored by: Netflix
Reviewed by: tsoome, kevans, emaste
Differential Revision: https://reviews.freebsd.org/D44056

(cherry picked from commit 5b3b9a58584b101e3ea0fd8dd50dc8d9157c058e)