Force -mtune=i386.  Random -mtune can lead to iPxeBiosBin.rom getting too
large (larger than 56k).  This can happen because we support custom CFLAGS.

--- a/src/VBox/Devices/PC/ipxe/Makefile.kmk
+++ b/src/VBox/Devices/PC/ipxe/Makefile.kmk
@@ -125,6 +125,7 @@
 	-ffreestanding \
 	-ffunction-sections \
 	-march=i386 \
+	-mtune=i386 \
 	-fomit-frame-pointer \
 	-fstrength-reduce \
 	-falign-jumps=1 \