assembler: Add XTheadCondMov support

[OFFTKP]

Useful extension for implementing conditional moves in a manner more similar to x86/ARM.
The standard Zicond way to do conditional movs is czero.nez + czero.eqz + or/add, while XTheadCondMov can do it in 1 instruction.

For example:

th.mveqz a0,a2,a1

versus

czero.nez a2,a2,a1
czero.eqz a0,a0,a1
add a0,a0,a2

GCC picks it over Zicond when both are available: https://godbolt.org/z/bfG7reh8W

Documentation is in here: https://github.com/XUANTIE-RV/thead-extension-spec/releases/tag/2.3.0

[lioncash]

Sorry about the delay reviewing this, it's been a little all over the place. Will get around to looking at this and (primarily) the spec docs tomorrow

[lioncash]

One minor nit, but this looks fine overall

[lioncash]

perhaps rename this file to assembler_xthead_tests.cpp so it makes adding further xthead tests a little more straightforward without needing to add more cpp files (mainly because I'll add support for the rest of the Xthead instructions in the assembler, so it saves me a little busywork).

[OFFTKP]

Some of the extensions in that spec are not useful (at least to me) in the presence of other official extensions, and they may even be breaking stuff.

For example, XTheadVector is a "non-standard non-conforming extension", I find no reason to use it when there's V, since (afaics) they don't interact with each other, meaning you can't use those instructions on the standard vector registers.

XTheadCondMov is useful to me because (imo) they sorta messed up with Zicond.

On the other hand, some other extensions might be useful even if there's better standard alternatives.
For example XTheadBa which defines ADDSL, which works like SH1/2/3ADD - but might be useful for chips that have XTheadBa but not Zba.

Thought I'd let you know, I'll do the rename.

[lioncash]

Alright, looks good!