toolchain: arcmwdt: add compile options: -Wno-typedef-redefinition

Without the -Wno-typedef-redefinition option, arcmwdt (clang based)
complains if a typedef gets redefined in gnu99  mode (since this is
officially a C11 feature). While new versions of GCC do not seem to
issue this warning in gnu99 mode anymore. So some existing code
with typedef redefined which works well with GCC will issue this
warning. As this warning is not fatal, so simply shut it off for good.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>