Change the priority of the XCVmem instructions

Created by: MaryBennett

Returned corev.md to be included at the bottom of riscv.md.

Issue #88 (closed) fix

Files Changed:

• corev.md: Added generic load/ store instructions with lower priority than the XCVmem load/ store instructions.
• riscv.md: Prevent generic load/ store instructions having higher priority than XCVmem load/ store if the extension is included.