Filter out invalid 'target' items from being passed to LLVM

Craig noticed that CodeGen wasn't properly ignoring the
values sent to the target attribute. This patch ignores
them.

This patch also sets the 'default' for this checking to
'supported', since only X86 has implemented the support
for checking valid CPU names and Feature Names.

One test was changed to i686, since it uses a lakemont,
which would otherwise be prohibited in x86_64.

Differential Revision: https://reviews.llvm.org/D39357

Change-Id: I2199e186a4133c1d596afb3b6f70a2a2e52436b4
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316783 91177308-0d34-0410-b5e6-96231b3b80d8