RISC-V: Split mstatus fields

What

Split mstatus fields further.

Does not impact performance as the interrupts cache already made an equivalent progress.

Closes RV-91

Why

Performance

How

Splitting fields of mstatus will allow directly accessing that field, without reading the other mstatus fields and skipping bit wrangling.

Performance

	master	This MR	Improvement
$MyMachine	0.412 TPS (1/873) native)	0.422 TPS (1/876 native)	+2.4%
Benchmark Machine	0.171 TPS (1/701 native)	0.169 TPS (1/739 native)	-1.2%

Barely noticeable in any direction.

Manually testing the MR

cd src/riscv && make test

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR