include/efi: Override EFIAPI macro for x86_64

This commit overrides the EFIAPI macro definition when using FSP on
x86_64 to ensure the correct calling convention is used.

On i386, there is no side-effect since the C calling convention used
by coreboot and FSP are the same. However, on x86_64, FSP/UEFI uses
the Microsoft x64 calling convention while coreboot uses the System
V AMD64 ABI.

This change resolves this incompatibility by setting EFIAPI to
attribute((ms_abi)) on x86_64 when using FSP.

TEST=Able to build google/rex0 in 32-bit and 64-bit mode.

Change-Id: Ifae910be66d550af04cce5136d186a7e9dd085b3
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82266
Reviewed-by: Ronak Kanabar <ronak.kanabar@intel.com>
Reviewed-by: Eric Lai <ericllai@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/efi/efi_datatype.h

@@ -3,6 +3,22 @@
 /* Create EFI equivalent datatype in coreboot based on UEFI specification */
 #ifndef __EFI_DATATYPE_H__
 #define __EFI_DATATYPE_H__
+
+/*
+ * EDK2 EFIAPI macro definition relies on compiler flags such as __GNUC__ which
+ * is not working well when included by coreboot. While it has no side-effect on
+ * i386 because the C calling convention used by coreboot and FSP are the same,
+ * it breaks on x86_64 because FSP/UEFI uses the Microsoft x64 calling
+ * convention while coreboot uses the System V AMD64 ABI.
+ *
+ * Fortunately, EDK2 header allows to override EFIAPI.
+ */
+#if CONFIG(PLATFORM_USES_FSP2_X86_32)
+#define EFIAPI __attribute__((regparm(0)))
+#else
+#define EFIAPI __attribute__((__ms_abi__))
+#endif
+
 #include <Base.h>
 #include <Uefi/UefiBaseType.h>