BootRules: Fix BuildMBR under Clang.

We need to force -m32 here, and we also need to tell Clang that we
want to allow relocations in .text.

Clang builds now get to kernel entry, but the bootloader fails to
load the early-boot modules with strange remapping errors, and so
panics with "get boot partitions failed!"

build/jam/BootRules

@@ -221,6 +221,6 @@ actions BuildMBR
 {
 	$(RM) $(1)
 	$(HAIKU_CC_$(HAIKU_PACKAGING_ARCH)) $(HAIKU_LINKFLAGS_$(HAIKU_PACKAGING_ARCH)) \
-		$(2) -o $(1) $(MBRFLAGS) -nostdlib -Xlinker --oformat=binary \
+		$(2) -o $(1) $(MBRFLAGS) -nostdlib -m32 -Wl,--oformat,binary -Wl,-z,notext \
 		-Xlinker -S -Xlinker -N -Xlinker --entry=start -Xlinker -Ttext=0x600
 }