KVM: SVM: Create a stack frame in __svm_vcpu_run() for unwinding

Unconditionally create a stack frame in __svm_vcpu_run() to play nice with
unwinding via frame pointers, at least until the point where RBP is loaded
with the guest's value.  Don't bother conditioning the code on
CONFIG_FRAME_POINTER=y, as RBP needs to be saved and restored anyways (due
to it being clobbered with the guest's value); omitting the "MOV RSP, RBP"
is not worth the extra #ifdef.

Creating a stack frame will allow removing the OBJECT_FILES_NON_STANDARD
tag from vmenter.S once __svm_sev_es_vcpu_run() is fixed to not stomp all
over RBP for no reason.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240223204233.3337324-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/vmenter.S

@@ -99,6 +99,7 @@
  */
 SYM_FUNC_START(__svm_vcpu_run)
 	push %_ASM_BP
+	mov  %_ASM_SP, %_ASM_BP
 #ifdef CONFIG_X86_64
 	push %r15
 	push %r14