riscv: Add macro for hfence.gvma instruction

Add an inline function hfence_gvma() for hfence.gvma instruction.

Signed-off-by: Doongar Singh <doonbsd@gmail.com>
Reviewed by: mhorne, markj
Differential Revision: https://reviews.freebsd.org/D54857

(cherry picked from commit 972c488bf4117d9eb29e5753c6c5a2250154df3d)