Add U-Boot x86_64 payload

Currently seems to stall when booted from the GRUB
payload, but works when booted from the SeaBIOS menu.

I also tested it as a standalone payload and it seems
to boot. Will test on hardware next, and start adding
it to more mainboards.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/u-boot/amd64coreboot/target.cfg

@@ -0,0 +1,5 @@
+tree="x86_64"
+# test building with x86_64 hostcc by commenting these:
+# xtree="default" # coreboot tree containing crossgcc
+# xarch="x86_64-elf"
+# or uncomment them to use crossgcc(buggy)

config/u-boot/x86_64/nuke.list

@@ -0,0 +1 @@
+test/lib/strlcat.c

config/u-boot/x86_64/target.cfg

@@ -0,0 +1,2 @@
+tree="x86_64"
+rev="3f772959501c99fbe5aa0b22a36efe3478d1ae1c" # v2024.07