Implement AArch64 ABI

[yuyichao]

This fixes a few SegFault in the test (similar to the ones on ARM) and make the ccall test pass.

A few notes about this PR,

• I do not include the LDC license since the header style and the link to the doc is the only thing I copied over. AFAICT, most of their ABI file is dealing with va_list and we don't have that yet.
• Most of the lines in abi_aarch64.cpp are comments copied from the calling convension doc, which I use when writing this PR. I think keeping those lines can help reviewing and other people reading the code.
• __fp16 (Float16) is allowed as C argument on AArch64 and is passed in a SIMD and floating-point register if possible (different from Int16).
• Adding byval attribute to a byRef argument breaks the code generation for that argument. See repro here. Note that in the working case the mov x0, sp correctly set the pass-by-pointer argument while in the broken case this instruction is missing. This is on LLVM 3.7.0 and I couldn't find any docs about how the byval attribute affects ABI or how is it processes in a target-dependent way so I'm not sure if this is an LLVM bug or if we are using it incorrectly (on AArch64 at least) (@Keno @vtjnash ). Clang doesn't emit such attribute for these arguments so I just leave it out.
• The alignment of __int128 in C is 16 bytes on AArch64 so I increase the MAX_ALIGN for AArch64. This also seems to be the case on x64 although I haven't find any document for this.
• Probably need tests for some corner cases that are important/special on this arch. Will try to add later.

[vtjnash]

i believe tests will be covered by #14215

since this is a strict improvement, i'm going to merge this now.

[JeffBezanson]

Nice!!

[yuyichao]

I believe tests will be covered by #14215

I was going to add some AArch64 specific test (__int128, __fp128, __fp16, arguments alignment and spilling) after figuring out other problems on ARM but yes, those can be added later if not covered by #14215.

[ViralBShah]

Can we backport this to support it on 0.4?

[yuyichao]

Should be similar to #14194. Might need #14688 and I should probably have a look at #14215 ...

[ViralBShah]

Putting the label on for now.