[committed][RISC-V] Fix minor issues in diagnostic message

This fixes two minor issues with the recently adding 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.

gcc/
	* config/riscv/riscv.cc (riscv_option_override): Spell out
	greater than and use cannot in diagnostic string.