Update to the 20190307 snapshot of GCC 7.4.1.

No longer require a not too old version of GCC to build on powerpc64, but
rely on the system compiler (even if that means we need to be explicitly
conservative when it comes to optimizations). [1]

PR: 235975 [1]
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl> [1]