linux: Implement pipe in terms of __NR_pipe2

The syscall pipe2 was added in linux 2.6.27 and glibc requires linux
3.2.0.  The patch removes the arch-specific implementation for alpha,
ia64, mips, sh, and sparc which requires a different kernel ABI
than the usual one.

Checked on x86_64-linux-gnu and with a build for the affected ABIs.