Enhanced llvm-link.

Added two flags to llvm-link to help reduce link time and thus shorten the annotate-compile-link cycle for SOAAP:

-dir: tells llvm-link to link all files in the directory specified as input argument one. The motivation for this comes due to the fact that the number of files that need to be linked to create chrome.bc is too many to pass as command-line arguments to llvm-link in one go (buffer size exceeds ARG_MAX); As a result I was having to do the linking in two stages wherein I linked the first half of files and then the second half. This meant that link times were quite long due to having to write bitcode twice. This flag means that all linking can happen in one step.

-disable-verify: once linking is done, llvm-link verifies the resulting linked module, which can add a further 10-15 seconds to link time. This verification probably isn't required when just adding/editing annotations, hence this flag allows you to eliminate this overhead.

With these two flags and using -Oz when compiling chrome source files, the time to link chrome.bc takes about 2 mins 36 secs on zenith.