remove cruft for supposedly-buggy clang from or1k & microblaze syscall_arch

remove cruft for supposedly-buggy clang from or1k & microblaze syscall_arch

it was never demonstrated to me that this workaround was needed, and
seems likely that, if there ever was any clang version for which it
was needed, it's old enough to be unusably buggy in other ways. if it
turns out some compilers actually can't do the register allocation
right, we'll need to replace this with inline shuffling code, since
the external __syscall dependency is being removed.