arc: make sure _start is in the beginning of .text section

This is important to have entry point in the beginning of .text section
because it allows simple loading and execution of U-Boot.

For example pre-bootloader loads U-Boot in memory starting from offset
0x81000000 and then just jumps to the same address.

Otherwise pre-bootloader would need to find-out where entry-point is. In
its turn if it deals with binary image of U-Boot there's no way for
pre-bootloader to get required value.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

arch/arc/cpu/u-boot.lds

@@ -13,6 +13,7 @@ SECTIONS
 	.text :	{
 		*(.__text_start)
 		*(.__image_copy_start)
+		arch/arc/lib/start.o (.text*)
 		*(.text*)
 	}
 

arch/arc/lib/Makefile

@@ -4,6 +4,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+head-y := start.o
 obj-y += cache.o
 obj-y += cpu.o
 obj-y += interrupts.o
@@ -18,7 +19,6 @@ obj-y += memcpy-700.o
 obj-y += memset.o
 obj-y += reset.o
 obj-y += timer.o
-obj-y += start.o
 obj-y += ints_low.o
 obj-y += init_helpers.o