The armclang shebang is required in the scatter file in order to build with CMake.

The heap size was incorrectly calculated.
This fixes it by subtracting the Stack size, any memory chunks allocated
before the start of the application (for vectors and/or crash report), and
finally the size of the application from the total RAM size.

Workaround a bug where the boot stack size configuration option is not
passed on to armlink, the Arm Compiler's linker. Prefer
MBED_CONF_TARGET_BOOT_STACK_SIZE if present, as this is what the
configuration system should provide. Fall back to MBED_BOOT_STACK_SIZE
if MBED_CONF_TARGET_BOOT_STACK_SIZE is not defined, as in the case of
buggy tools. If both MBED_CONF_TARGET_BOOT_STACK_SIZE and
MBED_BOOT_STACK_SIZE are not defined, then we fall back to a hard-coded
value provided by the linkerscript. See
https://github.com/ARMmbed/mbed-os/issues/13474 for more information.

To allow overriding of the boot stack size from the Mbed configuration
system, consistently use MBED_CONF_TARGET_BOOT_STACK_SIZE rather than
MBED_BOOT_STACK_SIZE.

Fixes #10319

ARM Compiler 6.13 testing revealed linker errors pointing out
conflicting use of `__user_setup_stackheap` and
`__user_initial_stackheap` in some targets. Remove the unwanted
`__user_initial_stackheap` from the targets - the setup is
centralised in the common platform code.

Looking into this, a number of other issues were highlighted

* Almost all targets had `__initial_sp` hardcoded in assembler,
  rather than getting it from the scatter file. This was behind
  issue #11313. Fix this generally.
* A few targets' `__initial_sp` values did not match the scatter
  file layout, in some cases meaning they were overlapping heap
  space. They now all use the area reserved in the scatter file.
  If any problems are seen, then there is an error in the
  scatter file.
* A number of targets were reserving unneeded space for heap and
  stack in their startup assembler, on top of the space reserved in
  the scatter file, so wasting a few K. A couple were using that
  space for the stack, rather than the space in the scatter file.

To clarify expected behaviour:

* Each scatter file contains empty regions `ARM_LIB_HEAP` and
  `ARM_LIB_STACK` to reserve space. `ARM_LIB_STACK` is sized
  by the macro `MBED_BOOT_STACK_SIZE`, which is set by the tools.
  `ARM_LIB_HEAP` is generally the space left over after static
  RAM and stack.
* The address of the end of `ARM_LIB_STACK` is written into the
  vector table and on reset the CPU sets MSP to that address.
* The common platform code in Mbed OS provides `__user_setup_stackheap`
  for the ARM library. The ARM library calls this during startup, and
  it calls `__mbed_user_setup_stackheap`.
* The default weak definition of `__mbed_user_setup_stackheap` does not
  modify SP, so we remain on the boot stack, and the heap is set to
  the region described by `ARM_LIB_HEAP`. If `ARM_LIB_HEAP` doesn't
  exist, then the heap is the space from the end of the used data in
  `RW_IRAM1` to the start of `ARM_LIB_STACK`.
* Targets can override `__mbed_user_setup_stackheap` if they want.
  Currently only Renesas (ARMv7-A class) devices do.
* If microlib is in use, then it doesn't call `__user_setup_stackheap`.
  Instead it just finds and uses `ARM_LIB_STACK` and `ARM_LIB_HEAP`
  itself.

targets/cmsis -> cmsis
targets/cmsis/TARGET_* -> targets/TARGET_*/device
targets/cmsis/TARGET_*/mbed_rtx.h -> targets/TARGET_*/mbed_rtx.h