src/arch/x86/exit_car: Fix regression on x86_64

The commit d023909b "treewide: Disable R_AMD64_32S relocation support"
clflush the address stored in _cbmem_top_ptr, which is the same address
cbmem_top() returns, instead of clflush _cbmem_top_ptr itself.

Fix that by providing the correct address to clflush.

Change-Id: If74591e7753cd9c3c097516430a212d416f53e4d
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59871
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/x86/exit_car.S

@@ -62,7 +62,7 @@ _start:
 	btl	$CPUID_FEATURE_CLFLUSH_BIT, %edx
 	jnc	skip_clflush
 #if ENV_X86_64
-	movabs	_cbmem_top_ptr, %rax
+	movabs	$_cbmem_top_ptr, %rax
 	clflush	(%rax)
 #else
 	clflush	_cbmem_top_ptr