x86/coco: Define cc_vendor without CONFIG_ARCH_HAS_CC_PLATFORM

After commit a9ef277 ("x86/kvm: Fix SEV check in
sev_map_percpu_data()"), there is a build error when building
x86_64_defconfig with GCOV using LLVM:

  ld.lld: error: undefined symbol: cc_vendor
  >>> referenced by kvm.c
  >>>               arch/x86/kernel/kvm.o:(kvm_smp_prepare_boot_cpu) in archive vmlinux.a

which corresponds to

  if (cc_vendor != CC_VENDOR_AMD ||
      !cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
            return;

Without GCOV, clang is able to eliminate the use of cc_vendor because
cc_platform_has() evaluates to false when CONFIG_ARCH_HAS_CC_PLATFORM is
not set, meaning that if statement will be true no matter what value
cc_vendor has.

With GCOV, the instrumentation keeps the use of cc_vendor around for
code coverage purposes but cc_vendor is only declared, not defined,
without CONFIG_ARCH_HAS_CC_PLATFORM, leading to the build error above.

Provide a macro definition of cc_vendor when CONFIG_ARCH_HAS_CC_PLATFORM
is not set with a value of CC_VENDOR_NONE, so that the first condition
can always be evaluated/eliminated at compile time, avoiding the build
error altogether. This is very similar to the situation prior to
commit da86eb9 ("x86/coco: Get rid of accessor functions").

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Message-Id: <20240202-provide-cc_vendor-without-arch_has_cc_platform-v1-1-09ad5f2a3099@kernel.org>
Fixes: a9ef277 ("x86/kvm: Fix SEV check in sev_map_percpu_data()", 2024-01-31)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/coco.h

@@ -10,13 +10,14 @@ enum cc_vendor {
 	CC_VENDOR_INTEL,
 };
 
-extern enum cc_vendor cc_vendor;
-
 #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
+extern enum cc_vendor cc_vendor;
 void cc_set_mask(u64 mask);
 u64 cc_mkenc(u64 val);
 u64 cc_mkdec(u64 val);
 #else
+#define cc_vendor (CC_VENDOR_NONE)
+
 static inline u64 cc_mkenc(u64 val)
 {
 	return val;