Support ILP32 ABI for AArch64

[sanxiyn]

ILP32 ABI (-mabi=ilp32) is an ABI for AArch64 analogous to x32 ABI for x86-64. See Linaro Wiki page and GCC doc for AArch64 Options. See #1339 for an issue about x32.

[sanxiyn]

This depends on having LLVM support. Unlike x32, whose support landed in 2014 (see Upstreaming x32 ABI support on llvm-dev), ILP32 is not in upstream LLVM at the moment.

The latest news I am aware of is ARM ILP32 Data model on llvm-dev posted in May 2016, where someone with Linaro mail address (therefore who ought to know) said "LLVM still doesn't support ILP32 for AArch64, and I'm not aware of any immediate plans to do so".

[sanxiyn]

This just in from llvm-dev: Upstreaming ILP32 support for AArch64.

[F001]

May I ask what is the next step to push it forward? I'd be very glad if I can do something to help. Can someone please provide some guidance?

[sanxiyn]

The first thing to do is to check the status of LLVM. Is the support merged? If so, when did it merge? (It is to check whether Rust's LLVM is recent enough.) How does one use the support? Etc.

[sanxiyn]

See rust-lang/rust#81455.

[sanxiyn]

Fixed.