Provide userspace versions of do_cpuid() and cpuid_count() on i386.

Some older compilers, when generating PIC code, cannot handle inline
asm that clobbers %ebx (because %ebx is used as the GOT offset
register). Userspace versions avoid clobbering %ebx by saving it to
stack before executing the CPUID instruction.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week