Raise the file descriptor limits when running compiletest

We already do this for libstd tests automatically, and compiletest runs into the
same problems where when forking lots of processes lots of file descriptors are
created. On OSX we can use specific syscalls to raise the limits, in this
situation, though.

Closes #8904
auto merge of #8943 : alexcrichton/rust/issue-8904, r=brson

We already do this for libstd tests automatically, and compiletest runs into the
same problems where when forking lots of processes lots of file descriptors are
created. On OSX we can use specific syscalls to raise the limits, in this
situation, though.

Closes #8904