On powerpcspe configure GCC --with-cpu=8548 instead of --with-cpu=8540,
which caused the build to fail after the update to binutils 2.31 and was
factually incorrect anyways (the oldest we support being 8548). [1]

On the way update to the 20180201 snapshot of GCC 8.2.1.

PR: 235393 [1]
Reported by: jhibbits [1]
Tested by: jhibbits [1]