Add x86_64-unknown-linux-gnux32 target

[malbarbo]

This adds X32 ABI support for Linux on X86_64. Let's package and dist it so we can star testing libc, libstd, etc.

Fixes rust-lang/rfcs#1339

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[malbarbo]

Add the target build to cross2 because it seems to have some free time.

[malbarbo]

Why this target is important?

Compared to amd64, x32 offers significant memory savings, often on the order of 30%, and modest efficiency gains.

Compared to i386, there's no memory use difference, but speed increases are more pronounced, especially in code that's under register pressure, operates on 64-bit or floating-point variables. It also avoids i386's penalty for PIC code, where EBX is essentially reserved for the Global Offset Table (GOT).

There are three principal use cases:

• vserver hosting (memory bound)
• netbooks/tablets (low memory, want performance)
• scientific tasks (want every % of performance)

From Debian X32 port.

[malbarbo]

r? @alexcrichton

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 7199fee has been approved by alexcrichton

[bors]

⌛ Testing commit 7199fee with merge 6c0d50f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 6c0d50f to master...