Support pre-processor macros and includes

[pelletier]

Hello! I've been trying to express CPU-feature guards with avo, using the new microarchitecture level groups. Here's an example from the Go standard library:

#ifndef hasAVX2
	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
	JEQ     big_loop_avx2
	JMP	big_loop
#else
	JMP	big_loop_avx2
#endif

https://github.com/golang/go/blob/f79c99fe8ae4a5e4380af22ee6cb38c3eb3a0416/src/internal/bytealg/compare_amd64.s#L48-L54

As far as I can tell, the main thing missing from Avo to achieve this today is the ability to emit calls pre-processor macros (#ifndef, #else, etc.) The other missing piece is to provide custom includes to the file to use the constants referenced in these macros, so I also added a helper for that.

I'm submitting this PR as a concrete basis to discuss achieving this capability. I'm sorry if I missed something, and this is already possible!

---

Other options I've considered:

Allow emitting arbitrary text. A function like RAW("#ifndef hasAVX2") would create a literal IR node which would be printed as-is. This felt too low-level compared to the rest of Avo. However, it would be nice to have this escape hatch.

Another option was to narrow the problem to this specific macro: #ifndef CPU feature. For example (fictitious Avo code):

import ".../avo/guards"

IFNOTGUARD(guards.HasAVX2)
// instructions if not present
ELSE()
// instructions if present
ENDIF()

It would provide a higher level abstraction, and avoid exposing includes. I think it would add too much complexity to Avo to validate the control flow of those expressions.

[klauspost]

LGTM. That would solve #235 for me.

[mmcloughlin]

Thanks for this PR! I've posted some thoughts on the issue #235.

[pelletier]

@mmcloughlin as discussed in #235, I've updated this PR to provide functions such as preproc.Ifndef, preproc.Else etc. as opposed to the original generic Preprocessor function. These functions insert their own node types in the IR. The intent is that the IR can then be checked for invalid constructions without having to do some string parsing of a generic node type.

I've also renamed Include to IncludeHeader to hopefully better convey that this function can be used anywhere to add a header, and is not a simple #include preprocessor macro insertion.