Fix #235 - Support aarch64 and non-GPU systems, tidy up gpuapi builds

[lars-t-hansen]

GPU API fixes:

• Introduce subdirectories for architecture-specific gpuapi libraries
• Make build.rs pick the right subdirectory
• Introduce ifdefs in the gpuapi library sources to allow them to be compiled even if GPU API is unavailable
• Introduce site and node specific build scripts for the libraries

Other aarch64 fixes:

• /proc/cpuinfo is completely different, so parse it properly
• cty::c_char is u8, so clean up the mess around C's char type

Feature system:

• Add Cargo features for amd, nvidia, xpu to complement the stub libraries
• Add a skeleton implementation of XPU that is not supposed to link yet (no libraries)
• Add test cases for feature combinations

Also ran cargo fmt and picked up some random changes that way.

Also cleaned up the text in some test cases.

[lars-t-hansen]

The test failure seems to be caused by the return of an old problem wherein the github runner environment is missing some environment variable that the tests depend on.

[lars-t-hansen]

Specifically, in #193 we fixed a regex so that user names could contain dashes, now we have a user name with a leading underscore.

[bast]

There is a conflict here - should I look at that?

[lars-t-hansen]

There is a conflict here - should I look at that?

If convenient, otherwise I'll deal with in the morning. If you're happy with the patch in other respects I can merge it myself, LMK.

[bast]

Thank you!