Log Message:
Use gsed when building GCC.

This is a somewhat blind commit. I've long had issues on various platforms
with libgcc getting misconfigured (on netbsd/mips,arm,powerpc), for example
build failures see:
http://mail-index.netbsd.org/port-mips/2017/06/21/msg000832.html

testing GCC 7.1 with netbsd/mips64el I got a lot further using it, but
still didn't complete the build. It took hours to reach this failure,
so I'd rather blindly commit the same change in the hopes it might help
other architectures.