x86/Makefile: Allow addition of link libraries for rom/ramstage

This is useful, for example, when using stage-independent code, as it
allows us to compile that code only once. It's also useful for vendor
code which needs wonky compiler definitions and include paths which
we'd rather not include in the other files.

Subsequent patches will make use of this when lib-izing AGESA.

Change-Id: Ifb0c5d353bf09d23864270b9eefb6b75fd86e6cb
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/5425
Reviewed-by: Aaron Durbin <adurbin@google.com>
Tested-by: build bot (Jenkins)