latest NaCl Mono changes - rebased

This is a rebase of pull request 538 to make it easier to review/commit: https://github.com/mono/mono/pull/538

Reproduced description from there:

This represents the last ~2 years of changes to NaCl Mono. The most notable change from the NaCl code that is in mono/mono right now is that this supports glibc and dynamic libraries, along with improved (boehm) GC support. It no longer supports newlib for x86/amd64.

NaCl also to some degree self-hosts with these changes. Until a recent merge, all of mcs was able to be compiled under NaCl Mono (this is now done by the host machine's Mono), and only gacutil.exe and mdoc.exe were required to build using the host machine's Mono. Many tests in mono/tests also run and pass in NaCl.

You may not want to take the nacl/ directory but those do include the scripts to build NaCl Mono in its current form. Further instructions are here: https://docs.google.com/a/google.com/document/d/1Jd_4M7mlmxF8daVbepAy_8RKYcRbhifXanRYyBKkVa4/pub