x86/traps: Rename compat_hypercall() to entry_int82()

x86/traps: Rename compat_hypercall() to entry_int82()

This follows the Linux example of naming the entry point by how it is arrived
at, rather than its purpose.

Doing so highlights that the SAVE_VOLATILE instantiation sets up the wrong
entry_vector on the stack (although this is currently benign as we never
sysret back to a 32bit PV, and the iret path doesn't care).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>