irgen: modify the ABI to use init guards instead of priority

The new ABI is simpler for use cases such as dynamically loaded packages.

The calling convention for import functions is similar to what go/ssa would
produce if BareInits were cleared. However, simply clearing this flag causes
two additional issues:

 1) We would need to special case the 'init$guard' variable (see
    discussion in https://codereview.appspot.com/78780043/).

 2) The call to __go_register_gc_roots needs to appear in the right
    place, i.e. after the guard check. Making this check appear
    in the right place with non-bare inits seems unreliable at best.

So we keep BareInits set and generate the necessary code manually.

It is still possible to get the old ABI by specifying a path to a gccgo
installation.

Differential Revision: http://reviews.llvm.org/D6804

llvm-svn: 225030

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