compiler must-haves for RISCV-android

[appujee]

we can fairly easily add extensions to the compiler driver's android-riscv64 target triples (at least until we add riscv64 to CTS/CDD!), but there's some more fundamental psabi-type work that needs to be finished before we can give anyone even a preliminary NDK:

• clang: check that the global clang driver's riscv64 default flags make sense for Android #9
• Fix RISC-V TLS bugs in llvm upstream #57
• Specify platform reserved register in clang driver (Relax gp could be platform specific register rather than reserved for… riscv-non-isa/riscv-elf-psabi-doc#371, Specify a platform reserved register riscv-non-isa/riscv-elf-psabi-doc#370)
• Atomic Atomics fixes riscv/riscv-isa-manual#992 Initial psABI atomics specification riscv-non-isa/riscv-elf-psabi-doc#378

(and anyone hacking together their own ndk before these are fixed should be aware that they'll have to rebuild everything at some point.)

[appujee]

Atomic fixes have been merged in the latest llvm-android. https://android-review.googlesource.com/c/toolchain/llvm_android/+/2631670

[enh-google]

we already have a separate bug for adding V to the driver, i think, and Zba/Zbs are already in (llvm, if not in aosp yet), so we can probably close this bug (unless you want to include Zba/Zbs/V here too)?

[appujee]

okay to close this i guess.