build: rename non-generated linker scripts to .ld extension

Avoids confusion with .gitignore rules, which were inadequate to
cover all the places where these files are found. At least in
VIM, these files are now syntax highlighted correctly.

Change-Id: I23810b0ed34129320cc2760e19ed1a610afe039e
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>