payload/external/edk2: Explicitly define the build arch as X64

Upstream commit 11ad164bce (UefiPayloadPkg: Make UPL build script arch
agnostic, 2024-02-22) changes the build script's behavior to not assume
the arch. Without defining BUILD_ARCH, the build script will not
function properly and results in the payload failing to build.

Both UefiPayload and Universal Payload can only be built in X64.

Change-Id: Icd942d0c15a99231d09f9cbdc5eb48333b6aa6e5
Signed-off-by: Nicholas Sudsgaard <devel+coreboot@nsudsgaard.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80883
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
diff --git a/payloads/external/edk2/Makefile b/payloads/external/edk2/Makefile
index 4cb7f0f..ef3327e 100644
--- a/payloads/external/edk2/Makefile
+++ b/payloads/external/edk2/Makefile
@@ -37,6 +37,8 @@
 endif
 endif
 
+BUILD_STR += -D BUILD_ARCH=X64
+
 #
 # EDK II (edk2/master) has the following build options relevant to coreboot:
 #