Enable linaro-gcc bootstrap with asan config.

Enable linaro-gcc bootstrap with asan config.

We couldn't verify linaro-gcc bootstrap with dynmaic ASan as we don't update
library pathes while running executables, so the compilation is performed with
the new instrumentations and the old external runtime from the build environment
is used during run.

And we'll also get runtime conflicts with 2 libasan.so version inside sanitized
environment, so it doesn't make sense to define 'asanbootstrap' with 'asan'.

If we really do want to test dynamic ASan, we could add the following to the
POSTSTAGE1_LDFLAGS:

  "-rpath$$r/prev-$(TARGET_SUBDIR)/libsanitizer/asan/.libs"

and keep additional rpathes for final binaries or use patchelf tool.

Change-Id: I8783f0395521126f8e01125e24bc40ed457bc008