Modify linker scripts to add heap to memory map, add list of supported c libs and "bare-metal" to supported_application_profiles.

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

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

### Description

The Mbed 2 release was broken for FF_LPC546XX as it would include
the entire mbed 2 build of the LPC546XX. This PR seperates these 2
builds completely by:

 * Removing non-shared extra labels from the MCU_LPC546XX target
 * Moving the target implementation to a shared target directory

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change