external-xilinx-toolchain: Enforce default hash style

* Some packages do not correctly handle LDFLAGS, as such toolchains that
  do not by default use GNU_HASH for symbols generated are not built
  with the GNU_HASH which causes a QA issue.
* Add the expected linker hash style to TARGET_CC_ARCH to enforce the
  specified hash style.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>