The legacy Cobalt tag sitting on the LSB part of the syscall code did
not cope well with the requirement for extending the numbering range,
for supporting the 32->64bit ABI models.

We fix this by reserving bit #28 as a marker for all Cobalt syscalls
(__COBALT_SYSCALL_BIT), which is still compatible with the I-pipe's
current syscall routing rules (i.e. call nr >= NR_syscalls being sent
unconditionally to the head domain).

For all archs, we now have:

nr = syscall_register & ~__COBALT_SYSCALL_BIT.

NOTE: when received, a Cobalt syscall has NOT been cleaned up from any
ABI mode tag bit which might belong to the host kernel,
e.g. __X32_SYSCALL_BIT may be present in syscall_register(orig_ax)
when received for handling an x32 call by a x86_64 host kernel.  Which
means that in the x32 case, we would have to filter out this extra bit
as well:

nr = syscall_register & ~(__X32_SYSCALL_BIT|__COBALT_SYSCALL_BIT).