KVM: x86: Enforce use of EXPORT_SYMBOL_FOR_KVM_INTERNAL

KVM: x86: Enforce use of EXPORT_SYMBOL_FOR_KVM_INTERNAL

Add a (gnarly) inline "script" in the Makefile to fail the build if there
is EXPORT_SYMBOL_GPL or EXPORT_SYMBOL usage in virt/kvm or arch/x86/kvm
beyond the known-good/expected exports for other modules.  Remembering to
use EXPORT_SYMBOL_FOR_KVM_INTERNAL is surprisingly difficult, and hoping
to detect "bad" exports via code review is not a robust long-term strategy.

Jump through a pile of hoops to coerce make into printing a human-friendly
error message, with the offending files+lines cleanly separated.

E.g. where <srctree> is the resolution of $(srctree), i.e. '.' for in-tree
builds, and the absolute path for out-of-tree-builds:

  <srctree>/arch/x86/kvm/Makefile:97: *** ERROR ***
  found 2 unwanted occurrences of EXPORT_SYMBOL_GPL:
    <srctree>/arch/x86/kvm/x86.c:686:EXPORT_SYMBOL_GPL(__kvm_set_user_return_msr);
    <srctree>/arch/x86/kvm/x86.c:703:EXPORT_SYMBOL_GPL(kvm_set_user_return_msr);
  in directories:
    <srctree>/arch/x86/kvm
    <srctree>/virt/kvm
  Use EXPORT_SYMBOL_FOR_KVM_INTERNAL, not EXPORT_SYMBOL_GPL.  Stop.

and

  <srctree>/arch/x86/kvm/Makefile:98: *** ERROR ***
  found 1 unwanted occurrences of EXPORT_SYMBOL:
    <srctree>/arch/x86/kvm/x86.c:709:EXPORT_SYMBOL(kvm_get_user_return_msr);
  in directories:
    <srctree>/arch/x86/kvm
    <srctree>/virt/kvm
  Use EXPORT_SYMBOL_FOR_KVM_INTERNAL, not EXPORT_SYMBOL.  Stop.

Put the enforcement in x86's Makefile even though the rule itself applies
to virt/kvm, as putting the enforcement in virt/kvm/Makefile.kvm would
effectively require exempting every architecture except x86.  PPC is the
only other architecture with sub-modules, and PPC hasn't been switched to
use EXPORT_SYMBOL_FOR_KVM_INTERNAL (and given its nearly-orphaned state,
likely never will).  And for KVM architectures without sub-modules, that
means that, barring truly spurious exports, the exports are intended for
non-KVM usage and thus shouldn't be using EXPORT_SYMBOL_FOR_KVM_INTERNAL.

Tested-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251121190514.293385-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>