Resolve ibtoolwrapper at the right path

Previously, bazel_tools would be under whatever.runfiles/workspace-name/external/bazel_tools. However, we allowed "" as the workspace name (and Bazel's workspace name was "") so a lot of scripts started assuming that bazel_tools would be at whatever.runfiles/external/bazel_tools.  

Going forward, the runfiles tree looks like:

whatever.runfiles/
    __main__/
       ...
    bazel_tools/
       tools/
         objc/
           ...
    some_other_repo/
       ...

So all of the repos are on the same "level" of the directory tree. This lets you refer to runfiles the same way (x.runfiles/repo/path/to/your/thing) whether you're running locally or importing it as an external repository (see https://github.com/bazelbuild/bazel/wiki/Updating-the-runfiles-tree-structure for more detail than you probably care about).

Fixes #1227.

--
MOS_MIGRATED_REVID=121435276

src/tools/xcode/actoolwrapper/actoolwrapper.sh

@@ -21,7 +21,7 @@
 
 set -eu
 
-MY_LOCATION=${MY_LOCATION:-"$0.runfiles/external/bazel_tools/tools/objc"}
+MY_LOCATION=${MY_LOCATION:-"$0.runfiles/bazel_tools/tools/objc"}
 REALPATH="${MY_LOCATION}/realpath"
 WRAPPER="${MY_LOCATION}/xcrunwrapper.sh"
 

src/tools/xcode/ibtoolwrapper/ibtoolwrapper.sh

@@ -25,7 +25,7 @@
 
 set -eu
 
-MY_LOCATION=${MY_LOCATION:-"$0.runfiles/external/bazel_tools/tools/objc"}
+MY_LOCATION=${MY_LOCATION:-"$0.runfiles/bazel_tools/tools/objc"}
 REALPATH="${MY_LOCATION}/realpath"
 WRAPPER="${MY_LOCATION}/xcrunwrapper.sh"
 

src/tools/xcode/momcwrapper/momcwrapper.sh

@@ -22,7 +22,7 @@
 
 set -eu
 
-MY_LOCATION=${MY_LOCATION:-"$0.runfiles/external/bazel_tools/tools/objc"}
+MY_LOCATION=${MY_LOCATION:-"$0.runfiles/bazel_tools/tools/objc"}
 REALPATH="${MY_LOCATION}/realpath"
 WRAPPER="${MY_LOCATION}/xcrunwrapper.sh"
 

src/tools/xcode/swiftstdlibtoolwrapper/swiftstdlibtoolwrapper.sh

@@ -21,7 +21,7 @@
 
 set -eu
 
-MY_LOCATION=${MY_LOCATION:-"$0.runfiles/external/bazel_tools/tools/objc"}
+MY_LOCATION=${MY_LOCATION:-"$0.runfiles/bazel_tools/tools/objc"}
 REALPATH="${MY_LOCATION}/realpath"
 WRAPPER="${MY_LOCATION}/xcrunwrapper.sh"