crossgcc: Update software versions

Update toolchain to the following software versions:

 o Python 3.5.1 -> 3.7.0
 o LLVM   6.0.0 -> 7.0.0
 o Expat  2.2.1 -> 2.2.5
 o MPC    1.0.3 -> 1.1.0
 o MPFR   3.1.5 -> 4.0.1

Change-Id: I66c6138c7b65c73a89b3cf980bb08950d8fffe6a
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/28887
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/crossgcc/sum/Python-3.7.0.tar.xz.cksum

@@ -0,0 +1 @@
+653cffa5b9f2a28150afe4705600d2e55d89b564  tarballs/Python-3.7.0.tar.xz