x86/local_apic.c: Thermal interrupt support: Additional style fixes

Rename handler function type 'lapic_thermal_handle_function' to the
shorter 'lapic_thermal_handler_t'. Move it closer to the function
declaration block where it is used. Make it a true function type (no
pointer) and add explicit pointer marks on usage.

Rename 'lapic_thermal_function_value' to the more immediately clear
'lapic_thermal_function_arg'. In lapic_thermal_enable(), use 'func_arg'
as the argument name for the handler argument, which at least refers to
function 'func', rather than the generic 'value'.

Finally, rename the global handler variable from
'lapic_thermal_function_ptr' to the shorter 'lapic_thermal_function'
(dynamic functions can be referenced only through a pointer).

MFC with: 87ba088fa310 ("x86/local_apic.c: Add support for installing a thermal interrupt handler")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit e1f4a8cb8656e64a1fe2b1ab519821b14c4985a0)