x86/mce: Mark mce_start() noinstr

Fixes

  vmlinux.o: warning: objtool: do_machine_check()+0x4ae: call to __const_udelay() leaves .noinstr.text section

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211208111343.8130-13-bp@alien8.de