[bugpoint] Increase the default memory limit for subprocesses to 300MB.

Summary:
Currently shared library builds (BUILD_SHARED_LIBS=ON in cmake) fail three
bugpoint tests (BugPoint/remove_arguments_test.ll,
BugPoint/crash-narrowfunctiontest.ll, and BugPoint/metadata.ll).

If I run the bugpoint commands that llvm-lit runs with without -silence-passes
I see errors such as this:
    opt: error while loading shared libraries: libLLVMSystemZInfo.so: failed to
    map segment from shared object: Cannot allocate memory

It seems that the increased size of the binaries in a shared library build is
causing the subprocess to exceed the 100MB memory limit. This patch therefore
increases the default limit to a level at which these tests pass.

Reviewers: dsanders

Reviewed By: dsanders

CC: llvm-commits, rafael

Differential Revision: http://llvm-reviews.chandlerc.com/D2013