cmake: disable plugins using isa-l on osx

cmake: disable plugins using isa-l on osx

clang on osx emits functions with leading underscore, but the isa-l
assembly's functions have no leading underscore. we could label the
function declaration like `int foo asm("foo")` to remove the leading
underscore, but isa-l is a git submodule, let do this later. and in the
meanwhile, disable this plugin on osx.

Signed-off-by: Kefu Chai <kchai@redhat.com>