Import the latest CloudABI definitions, v0.18.

In addition to some small style fixes to the ARMv6 vDSO, this release
includes a new vDSO that can be used for the execution of ARMv6/ARMv7
code on 64-bit platforms.

Just like for i686 on x86-64, this new vDSO is responsible for padding
arguments and return values to 64-bit values, so that the kernel can
easily forward system calls to the native system calls.

Obtained from: https://github.com/NuxiNL/cloudabi