[nolibc] Change internal syscall API to remove reliance on libc's errno.

This change moves to a model where the error value of a system call is
potentially contained in the return value itself rather than being
implicit in errno.  The helper function internal_iserror can be used
to extract the error value from a return value.  On platforms other
than Linux/x86_64 this still uses errno, but other platforms are free
to port their error handling to this new model.

Differential Revision: http://llvm-reviews.chandlerc.com/D756