lib: sbi_trap: Add helper to get GVA in sbi_trap_regs

The GVA bit is in mstatus on RV64, and in mstatush in RV32. Refactor
code handling this in sbi_trap_handler into a helper function to extract
GVA from sbi_trap_regs, so that future code accessing GVA can be
XLEN-agnostic.

Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>