Utilize existing assembler such as clang instead of nasm

[theoparis]

I would like to remove the dependency on NASM by using my toolchain's clang binary, but rav1e's build.rs seems to only support nasm 😕 Is there something blocking this from being implemented?

Edit: Attempting to use the cc crate to compile the .asm files strangely results in:

[lu-zero]

The x86_64 asm is written for nasm, a preprocessor or reimplementing the syntax nasm is using would be needed.

[mikerobi]

Rust now supports inline assembly, that may be a better option.

[negge]

See @tmatth comment below. We import these files as-is from dav1d and we should not be in the business of converting 200k+ lines of assembly source code just to avoid a nasm dependency.

[tmatth]

Files like src/x86/itx_avx2.asm were imported from dav1d and so I don't imagine we'd be changing toolchains in a way that would break that workflow (as we will continue to pull in asm as it gets improved/added to in dav1d).