treewide: rework linker symbol declarations in sections header

1. Convert all linker symbols to char[] type so that we can get the
   corresponding address by calling array name 'var' or its address
   '&var'. In this way, we can avoid some potential issues[1].
2. Remove unused symbol '_TEXT_BASE'. It has been abandoned and has
   not been referenced by any source code.
3. Move '__data_end' to the arch x86's own sections header as it's
   only used by x86 arch.
4. Remove some duplicate declared linker symbols. Now we use the
   standard header file to declare them.

[1] This patch fixes the boot failure on MIPS target. Error log:
SPL: Image overlaps SPL

Fixes: 1b8a1be1a1 ("spl: spl_legacy: Fix spl_end address")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

arch/microblaze/include/asm/processor.h

@@ -6,11 +6,6 @@
 #ifndef __ASM_MICROBLAZE_PROCESSOR_H
 #define __ASM_MICROBLAZE_PROCESSOR_H
 
-/* References to section boundaries */
-
-extern char _end[];
-extern char __text_start[];
-
 /* Microblaze board initialization function */
 void board_init(void);