perf test: Fix skipping branch stack sampling test

Commit f4a2aade ("perf tests powerpc: Fix branch stack sampling test
to include sanity check for branch filter") added a skip if certain
branch options aren't available. But the change added both -b
(--branch-any) and --branch-filter options at the same time, which will
always result in a failure on any platform because the arguments can't
be used together.

Fix this by removing -b (--branch-any) and leaving --branch-filter which
already specifies 'any'. Also add warning messages to the test and perf
tool.

Output on x86 before this fix:

   $ sudo ./perf test branch
   108: Check branch stack sampling         : Skip

After:

   $ sudo ./perf test branch
   108: Check branch stack sampling         : Ok

Fixes: f4a2aade ("perf tests powerpc: Fix branch stack sampling test to include sanity check for branch filter")
Signed-off-by: James Clark <james.clark@arm.com>