[misa] Clearing U does not clear dependent S

[kkapapu]

Environment

• NutShell: commit 0b3d3a86feb1e0c7c10f446a460443ad2d696e6d
• ISA: RV64 (NutShell default)

Describe the bug

misa initially has both User (U, bit 20) and Supervisor (S, bit 18) set. Clearing only U should also clear S because S depends on U. NutShell leaves misa unchanged (both bits still set).

Minimal trigger:

csrr s0, misa
li   t0, (1 << 20)    # U
csrc misa, t0         # clear U while S is still 1
csrr s1, misa         # mismatch here

Observed values:

Stage	Value	Meaning
Initial misa	0x8000000000141105	MXL=RV64, extensions include U|S|M|I|C|A
After csrc (Sail)	0x8000000000001105	U and S both cleared → M|I|C|A
After csrc (NutShell)	0x8000000000141105	U and S both still set (write ineffective w.r.t. dependency)

Mismatch at pc = 0x8000000c on s1:

• riscv-Sail (right): 0x8000000000001105
• NutShell (wrong): 0x8000000000141105

Expected behavior

Per RISC-V Privileged Spec (machine.adoc — misa extension dependencies):

If an ISA feature x depends on an ISA feature y, then attempting to enable feature x but disable feature y results in both features being disabled.
For example, setting "F"=0 and "D"=1 results in both "F" and "D" being cleared.
Similarly, setting "U"=0 and "S"=1 results in both "U" and "S" being cleared.

Clearing U while S remains set is exactly that illegal combination: the write must clear both U and S.

	After csrc misa, (1<<20)
Expected (Sail)	misa = 0x8000000000001105 (U=0, S=0)
Actual (NutShell)	misa = 0x8000000000141105 (U=1, S=1 unchanged)