From C you can simply
((void (*__near)(void))0xFFFE)();
, where __near is pointer type modifier, which you can remove. No matter what memory model, it still will work for S12 and S08.
There's no LDHX instruction in S12. Also ,X addressing mode is not supported. You need to modify your code like this
__asm LDX #$FFFE // reset vector
__asm JMP 0,X / /Jump and reset!