Re-organize software rasterizer code

file_sys: change RomFS archive to Self NCCH archive

Small VideoCore cleanups

Fixes #2455

The resulting pointer wasn't written to unless the index was verified as
valid, but that's still UB and triggered debug checks in MSVC.

Reported by garrettboast on IRC

This eliminates UB when aliasing it with the array of u32 regs, and
is compatible with non-LE architectures.

All enums that are used with BitField must have their type forced to u32
to ensure correctness.

This copy was left behind when the shader generation code was moved to a
separate file.

This gets rid of the static unordered_map. Also changes the return type
const char*, avoiding unnecessary allocations (the result was only used
by calling .c_str() on it.)

Also remove some unused members.

Split up monolithic Regs struct

externals: nihstro, update to latest master. Again.

Use std::array<u8,2> instead of u8[2] to fix MSVC build

IPC helper

Oh No! More shader changes!