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.