[llvm] [lit] Add target-x86* features

Add a 'target-x86' and 'target-x86_64' feature sthat indicates that
the default target is 32-bit or 64-bit x86, appropriately.  Combined
with 'native' feature, we're going to use this to control x86-specific
LLDB native process tests.

Differential Revision: https://reviews.llvm.org/D60474

llvm-svn: 358177