objtool: Make unwind hint definitions available to other architectures

Unwind hints are useful to provide objtool with information about stack
states in non-standard functions/code.

While the type of information being provided might be very arch
specific, the mechanism to provide the information can be useful for
other architectures.

Move the relevant unwint hint definitions for all architectures to
see.

[ jpoimboe: REGS_IRET -> REGS_PARTIAL ]

Signed-off-by: Julien Thierry <jthierry@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>