Rewrite the vDSOs for CloudABI in assembly.

Rewrite the vDSOs for CloudABI in assembly.

The reason why the old vDSOs were written in C using inline assembly was
purely because they were embedded in the C library directly as static
inline functions. This was practical during development, because it
meant you could invoke system calls without any library dependencies.
The vDSO was simply a copy of these functions.

Now that we require the use of the vDSO, there is no longer any need for
embedding them in C code directly. Rewriting them in assembly has the
advantage that they are closer to ideal (less useless branching, less
assumptions about registers remaining unclobbered by the kernel, etc).
They are also easier to build, as they no longer depend on the C type
information for CloudABI.

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