[llgo] build llgoi by default

Summary:
As llgoi is installed, it must be built or the install
target will fail; there is no implicit dependency.
Adding llgoi to "ALL" makes building llgo as part of an
entire LLVM distribution a bit friendlier: you can then
just do the typical "make && make install".

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11291

llvm-svn: 242750