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: 1b8a1be1a1f1 ("spl: spl_legacy: Fix spl_end address")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/freescale/ls1021atsn/ls1021atsn.c b/board/freescale/ls1021atsn/ls1021atsn.c
index d144f25..d0e4e79 100644
--- a/board/freescale/ls1021atsn/ls1021atsn.c
+++ b/board/freescale/ls1021atsn/ls1021atsn.c
@@ -12,6 +12,7 @@
 #include <asm/arch/ls102xa_soc.h>
 #include <asm/arch/fsl_serdes.h>
 #include <asm/global_data.h>
+#include <asm/sections.h>
 #include <linux/delay.h>
 #include "../common/sleep.h"
 #include <fsl_validate.h>