arm: move _end to linker symbols

commit 6e2228fb ("Merge patch series "Clean up arm linker scripts")
was cleaning up linker scripts for armv7 and v8 but was leaving
_end and __secure_stack_start/end.

commit d0b5d9da ("arm: make _end compiler-generated")
was moving _end to be compiler generated. _end is defined as c variable
in its own section to force the compiler emit relative a reference.
However, defining those in the linker script will do the same thing
since [0].

So let's remove the special sections from the linker scripts, the
variable definitions from sections.c and define them as a symbols.
It's worth noting that _image_binary_end symbol is now redundant and
can be removed in the future.

- SPL

The .end section has been removed from the new binary
[ 5] .end
     PROGBITS         00000000fffdf488  000000000002f488  0
     0000000000000000 0000000000000000  0                 1
     [0000000000000003]: WRITE, ALLOC

$~ bloat-o-meter kria_old/spl/u-boot-spl krina_new/spl/u-boot-spl
add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
Function                                     old     new   delta
Total: Before=115980, After=115980, chg +0.00%

$~ readelf -sW kria_old/u-boot kria_new/u-boot | grep -w _end
 12047: 000000000813a0f0     0 OBJECT  GLOBAL DEFAULT   11 _end
 12047: 000000000813a118     0 NOTYPE  GLOBAL DEFAULT   11 _end

$~ readelf -sW kria_old/spl/u-boot-spl kria_new/spl/u-boot-spl | grep -w _end
  1605: 00000000fffdf488     0 OBJECT  GLOBAL DEFAULT    5 _end
  1603: 00000000fffdf498     0 NOTYPE  GLOBAL DEFAULT    4 _end

$~ readelf -sW old/u-boot new/u-boot | grep -w _end
  8847: 0000000000103710     0 OBJECT  GLOBAL DEFAULT   11 _end
  8847: 0000000000103738     0 NOTYPE  GLOBAL DEFAULT   11 _end

$~ readelf -sW old_v7/u-boot new_v7/u-boot | grep -w _end
 10638: 000da824     0 OBJECT  GLOBAL DEFAULT   10 _end
 10637: 000da84c     0 NOTYPE  GLOBAL DEFAULT   10 _end

- For both QEMU instances
$~ bloat-o-meter old/u-boot new/u-boot
add/remove: 0/0 grow/shrink: 1/0 up/down: 20/0 (20)
Function                                     old     new   delta
version_string                                50      70     +20
Total: Before=656915, After=656935, chg +0.00%

[0] binutils commit 6b3b0ab89663 ("Make linker assigned symbol dynamic only for shared object")

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>