Apply --symlink_prefix to the output and exec root symlinks.

Turns out --symlink_prefix is not applied to the bazel-{out,workspace}
unlike bazel-{bin,genfiles,testlogs} thus breaking the semantics of
--symlink_prefix.

This CL fixes the problem by using the symlink prefix instead of the
product name when building the output and exec root symlinks.

The bazel-out symlink is still created for backwards compatiblity but
will be removed eventually.

RELNOTES[INC]: Using --symlink_prefix is now applied to the output
symlink (e.g. bazel-out) and the exec root symlink (e.g. bazel-workspace).

--
PiperOrigin-RevId: 147161547
MOS_MIGRATED_REVID=147161547

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