Add a pnacl_irt_shim.gyp to build the pnacl IRT shim with nacl-gcc.

Add a pnacl_irt_shim.gyp to build the pnacl IRT shim with nacl-gcc.
This is used by PNaCl, so we will need to figure out how to hook this
up to PNaCl despite it being built by nacl-gcc.

This is not yet activated.

There is a NaCl repo change as well to add some support scripts / and a 'generated_source' variable before this is activated.


BUG= http://code.google.com/p/nativeclient/issues/detail?id=2465
TEST= manually combine this with http://codereview.chromium.org/8817001/ and try to build the library in Visual Studio + via Make on Linux.