uml: use generic BUG

Get UML to use the generic bug support rather than arch specific one.

If I insert an artificial bug right before loading init, I get this:

 Kernel panic - not syncing: Kernel mode signal 4

 EIP: 0023:[<0819d501>] CPU: 0 Not tainted ESP: 002b:f7fd4fbc EFLAGS: 00000246
    Not tainted
    EAX: 00000000 EBX: 00007870 ECX: 00000013 EDX: 00007870
    ESI: 0000786d EDI: 00000011 EBP: f7fd4fd8 DS: 002b ES: 002b
    08273bec:  [<0806e814>] show_regs+0x104/0x106
    08273c08:  [<08058927>] panic_exit+0x2c/0x4b
    08273c18:  [<08080ee7>] notifier_call_chain+0x32/0x5b
    08273c38:  [<08080fbd>] __atomic_notifier_call_chain+0x30/0x32
    08273c54:  [<08080fee>] atomic_notifier_call_chain+0x2f/0x31
    08273c70:  [<08073b88>] panic+0x75/0x131
    08273c94:  [<080586c7>] relay_signal+0x87/0x95
    08273cb0:  [<0806b9ee>] sig_handler_common_skas+0x9e/0x120
    08273cd8:  [<08067738>] sig_handler+0x28/0x4f
    08273cec:  [<0806792e>] handle_signal+0x53/0x89
    08273d0c:  [<08069f60>] hard_handl...