Fix and simplify field lookup in SelectorExpr

(Was broken when struct contained multiple anonymous
fields, and selected field was in >0'th anonymous
field.)
llgo-dist: Enable build tags in .ll files

This commit introduces a new package, github.com/axw/llgo/build,
which contains functionality related to go/build. To start with,
it contains an type implementing io.ReadCloser that converts
LLVM IR comments to "//" style comments, so build tags can be
supported in .ll files.

Also changed the "getPackage" function to have go/build locate
.ll files by overriding ReadDir/OpenFile in go/build Context.
The provided functions will yield .ll files wherever they exist
with the same path prefix (everything before the suffix) as a
.s file. Thus, go/build handles all .ll files (having been
tricked into thinking they're .s files), checking filenames and
build tags.

Removed goodOSArchFile, as it's no longer required.