build: Allow clang build linkage to use libcompiler-rt

Make use of '-print-librt-file-name' over '-print-libgcc-file-name'
to use Compiler-RT runtime glue over libgcc glue.

NOTE: *** Requires at least clang 3.6.x

Change-Id: I7f63284473d6067bf775409970c8dd98f5d5a8d5
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6144
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)
cpu/x86/smm/Makefile.inc: Correct SMM linkage under clang builds

Ensure that the SMM build stage links properly using the compiler-rt
runtime under a Clang build.

Change-Id: Iead28c46d63f5bbb27757b7dc66fe06b4813d03c
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6462
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>