autotools.bbclass: libtool sysroot support changes
We do not do it for native recipes, as the native compiler should
fall back to a prefix of /usr and not solely depend on the sysroot.
Otherwise we end up staging everything in the native sysroot before
we start to build target recipes.
Also remove la mangling code, which is no longer necessary.
Commit derived from Khem Raj's OE commits.
Signed-off-by: Scott Garman <scott.a.garman@intel.com>
