RISC-V: Improve page table walker spec compliance

RISC-V: Improve page table walker spec compliance

- Inline PTE_TABLE check for better readability
- Change access checks from ternary operator to if
- Improve readibility of User page U mode and SUM test
- Disallow non U mode from fetching from User pages
- Add reserved PTE flag check: W or W|X
- Add misaligned PPN check
- Set READ protection for PTE X flag and mstatus.mxr
- Use memory_region_is_ram in pte update

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>