openrisc: Add gcc machine instruction flag configuration

OpenRISC GCC supports flags to enable the backend to output instructions
if they are supported by a target processor.  This patch adds
configuration flags to enable configuring these flags to tune the kernel
for a particular CPU configuration.

In the future we could also enable all of these flags by default and
provide instruction emulation in the kernel to make these choices easier
for users but this is what we provide for now.

Signed-off-by: Stafford Horne <shorne@gmail.com>