ifuncs: Include <machine/ifunc.h> instead of <x86/ifunc.h>

[olce]

Summary

All architectures have been providing an 'ifunc.h' header with
DEFINE_IFUNC() et alter working both in kernel and userland since commit
cf41d1113377 ("riscv: implement kernel ifunc resolution"), and separate
<i386/ifunc.h> and <amd64/ifunc.h> headers were introduced in commit
2b1db07bec92 ("x86: add machine/ifunc.h"), so stop including
<x86/ifunc.h> directly and use the common <machine/ifunc.h> idiom.

While here, re-order includes in the blocks featuring <x86/ifunc.h>,
which requires fixing <x86/apicvar.h> so that it can be included before
<machine/intr_machdep.h>.