powerpc: Stop pretending we run on e500v1 cores

powerpc: Stop pretending we run on e500v1 cores

Unconditional writing to MAS7, which doesn't exist on the e500v1 core, in a
TLB miss handler has been in the code for several years now.  Since this has
gone unnoticed for so long, it's easily concluded that e500v1 is not in use
with FreeBSD.  Simplify the code path a bit, by unconditionally zeroing MAS7
instead of calling a subroutine to do it.