clang driver changes that must happen before even an initial NDK

[enh-google]

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:

[ ] forward-compatible atomics sequences : #73
[ ] reserve a register for SCS : #72

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

[enh-google]

(dupe of #81.)