This fixes two minor issues with the recently added warning about too 
large VLEN in the RISC-V backend.  These prevent the RISC-V port from 
bootstrapping as both Andreas and I have found.

Specifically we'll get warnings for the use of '>' in the recently added 
message as well as using "can not" vs "cannot".  While these warnings 
may seem annoying, they're in place to make it easier for the translators.

This patch fixes the message in the fairly obvious way.  Spells out the 
greater than and uses cannot.  There's a similar issue in another 
recently added diagnostic that I'll push momentarily.

Committed to the trunk,
Jeff