x86: efi: Tweak the code used for the 64-bit EFI app

Add an empty CPU init function to avoid fiddling with low-level CPU
features in the app. Set up the C runtime correctly for 64-bit use
and avoid clearing BSS, since this is done by EFI when U-Boot is loaded.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/x86_64/cpu.c b/arch/x86/cpu/x86_64/cpu.c
index a3674e8..6a38761 100644
--- a/arch/x86/cpu/x86_64/cpu.c
+++ b/arch/x86/cpu/x86_64/cpu.c
@@ -45,3 +45,8 @@
 {
 	return CONFIG_CPU_ADDR_BITS;
 }
+
+int x86_cpu_init_f(void)
+{
+	return 0;
+}