Makefile: test for -fno-asynchronous-unwind-tables

Some GCC variants don't support this. Moved later in the Makefile
after the toolchain Makefile has been included so the test works
properly.

Change-Id: Ia9ec2582c486197ea3df3d878173643c14786408
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>