buildgcc: Update LLVM to 3.7.1

Not much testing, update mostly so we can test with the
latest scan-build.

Change-Id: I50d28b7e0dfd31f9ae565c8515d5ab1760ca4c62
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/13516
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/crossgcc/sum/cfe-3.7.1.src.tar.xz.cksum

@@ -0,0 +1 @@
+15475a2c1e750a13755812785a4796cf4d9fa4c2  tarballs/cfe-3.7.1.src.tar.xz