Support go 1.5

The go compiler and linker changed in v1.5 -- to 'go tool compile' and
'go tool link' instead of 6g and 6l. Move the selection logic to
bootstrap.bash, and have it use compile/link if 6g/6l are missing. This
way the build.ninja.in will continue working with either go 1.4 or 1.5.

Travis and the test suite will fail under 1.5, since the build.ninja.in
is still generated with 1.4, and the function names in the comments
differ between 1.4 and 1.5.