`c.nop` encoding is different than riscv-opcodes

[jordancarlin]

Current value and location
c.nop.yaml has a match field with a fully constrained value of 0000000000000001

Expected value
riscv-opcodes encodes c.nop as 000?00000?????01. Note the wildcards.

It looks like the divergence is because of how hints are handled. From the spec:

The C.NOP code points with imm≠0 encode HINTs.

Discovered while comparing #1090 to the riscv-opcodes version of the header.

[ThinkOpenly]

Also in the spec:

HINTs are executed as no-ops on implementations that ignore them.

So, it probably makes sense to accommodate them as wildcarded no-ops.