Made the Cortex-A12, A15 and A17 examples link with an LLVM toolchain (#596)

Those three sample scripts compiled crt0.S and reset.S but named neither on the
link line, and omitted -nostartfiles, so the toolchain was also free to link its
own startup. Under GNU that produced a working image; under an LLVM toolchain
picolibc's crt0 was pulled in and wanted __data_start, __data_source,
__data_size, __bss_size, __stack, __tls_base and __arm32_tls_tcb_offset, none of
which the linker script defines.

Defining picolibc's contract in the shared linker script was tried first and
abandoned: it reaches into ABI constants that cannot be verified here, and it is
unnecessary, because the in-tree crt0.S is already a complete startup for this
script. It sets up the stack and zeroes BSS between __bss_start__ and
__bss_end__, and .data carries no AT() so there is nothing to copy.

So the three scripts now pass -nostartfiles and name crt0.o and reset.o, which
is what the four sibling A profile cores already do and what these scripts were
evidently compiling those files for.

Both the old and the new GNU images contain the in-tree startup, __vectors from
reset.S and _mainCRTStartup from crt0.S, so the startup was already being used
through implicit startup file resolution rather than an explicit operand. How
that resolution happened without the objects being named is not accounted for
here, which is itself the argument for naming them: the same implicit behaviour
does not hold across toolchains, and that is why the LLVM link failed.

Add a readme to each of the three example directories. Their
tx_initialize_low_level.S is the generic ARMv7-A skeleton, 300 lines and
identical across all three, with no interrupt controller programming, no timer
and no vector table installation, where the A5, A7, A8 and A9 examples have all
three and ship the matching Versatile Express support files. These three
therefore demonstrate that the port builds; they will not receive a timer tick.
Nothing in the tree said so, which invites the assumption that they are
equivalent.

Verified with both toolchains for all three cores. GNU links at 41,276 bytes of
text, down from 41,768 because the unused toolchain startup is no longer
included, and Arm Toolchain for Embedded links at 37,982 where it previously
could not link at all. The resulting image is structurally sound: entry at
_start, __vectors at address zero, and a BSS range in RAM. It has not been
executed. scripts/check_clang.sh now links eleven of eleven example builds.

Assisted-by: Claude Code (Opus 5) <[email protected]>

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