Import vDSO-related source files from the CloudABI repository.

CloudABI executables that are emulated on Mac OS X do not invoke system
calls through "syscall". Instead, they make use of a vDSO that is
provided by the emulator that provides symbols for all of the system
call routines. The emulator can implement these any way it likes.

At some point in time we want to do this for native execution as well,
so that CloudABI executables are entirely oblivious of how system calls
need to be performed. They will simply call into functions and let that
deal with all of the details.

These source files can be used to generate a simple vDSO that does
nothing more than invoke "syscall". All we need to do now is map it into
the processes.

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