Android riscv64 support

[SiFiveHolland]

These changes allow building LLVM, including AMDGPU, for a riscv64 board.

[maurossi]

Hi, sorry for very late response

In this repo I don't process Pull Request, because I use it as my development repo and not as an upstream repo, also due to my hack approach "DO NOT MERGE: android: translate soong build rules for libLLVM18"

I have tested the build with your proposed change and it can build, but only with Android 14 and later where android_riscv64: property is supported.

I don't know how to implement a conditional rule based on Android version in Android.bp, therefore these changes will have to stay out-of-this-tree, because I am using it to build Android 11, 12, 13

As a side note llvm-project is still using __riscv__ as preprocessor definition

maurossi@ej-desktop:~/u-x86_michael_ax86/external/llvm-project$ cgrep __riscv__
./compiler-rt/lib/fuzzer/FuzzerTracePC.cpp:138:#elif defined(__riscv__)

[SiFiveHolland]

Alright, thanks for the explanation. It's no problem. I just wanted these changes to be available in case they were useful to others, and your repository had some visibility on the mesa issue tracker.