Several tests in gcc.target/arm use dg-options with -mcpu=xxxx, which
causes compiler warnings or errors when the multilib flags include
-march=yyyy.  This patch causes those tests to be skipped.  It also
prevents gcc.target/arm/20090811-1.c from running with multilibs that
would override -mcpu or -mfloat-abi options specified for the test.

Tested on arm-none-linux-gnueabi for several multilibs; these tests
are compile-only so it didn't matter that I didn't have runtime
support for them.

OK for trunk and 4.6 branch?  (This is just the tip of the iceberg.)