Use label syntax that is immune to MSYS path mangling in bootstrap scripts.

On Windows, MSYS will mangle all arguments that resemble Unix paths
when executing (exec*()) non-msys executables (in an attempt to convert
them to Windows paths). This affects ``//src:bazel`` (it becomes
``/src:bazel``) but not ``src:bazel``. This CL converts to the latter in
bootstrapping shell scripts to work around this issue.

--
MOS_MIGRATED_REVID=113349821

compile.sh

diff --git a/compile.sh b/compile.sh
index 9f435e0..880555d 100755
--- a/compile.sh
+++ b/compile.sh
@@ -89,7 +89,7 @@
   new_step 'Building Bazel with Bazel'
   display "."
   log "Building output/bazel"
-  bazel_build //src:bazel
+  bazel_build src:bazel
   cp -f bazel-bin/src/bazel output/bazel
   chmod 0755 output/bazel
   BAZEL=$(pwd)/output/bazel