cmd/llgo-dist: Add -x parameter for printing commands before running
cmd/llgo-dist: darwin specific changes.

- Added an option to use install_name_tool to set the absolute path of
libLLVM-xxx.dylib.
- pnacl_sdk does not keep 64 bit binaries in bin64 for darwin it
appears
cmd/llgo-dist: Added -v parameter for verboseness (program output)
runtime: add LLGO_ASM_EXPORT macro + dummy darwin files.

This makes llgo-dist and many (but not all) of the llgo unit tests
happier on my MBP.
.gitignore: add llgo-dist and llgo-build
llgo/utils_test.go: don't invoke clang with -g on Darwin.

Don't know if this is darwin specific or just specific to my system,
but the change is harmless enough to make in by default and revisited
at a later date.