[AAPCS64] Errata with callee/caller-saved SIMD registers specification.

[lemon32767]

In 6.1.2 SIMD and Floating-Point Registers,

Registers v8-v15 are Callee-saved and the remaining registers (v0-v7, v16-v31) are Callee-saved. Additionally, only the bottom 64 bits of each value stored in v8-v15 need to be Callee-saved [8]; it is the responsibility of the caller to preserve larger values.

I believe that should say:

Registers v8-v15 are Callee-saved and the remaining registers (v0-v7, v16-v31) are Caller-saved.

Introduced with the rewording from #371.

[smithp35]

Thanks for spotting the problem. I've created #386 to fix.