These are not used on Mbed OS. Remove them to avoid name conflict with other modules.

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

This is to support custom boards based on M261 series chips.

Add a "used" attribute to __vector_handlers to fix ARMC6 build with
the "-flto" flag.
(Error: L6236E: No section matches selector - no section to be FIRST/LAST. )

This attribute, attached to a function/variable, means that code must be emitted
for the function even if it appears that the function is not referenced.

On IAR, configure heap to 1KiB at a minimum and expandable, dependent on available SRAM. This requires IAR 8.x.

Support targets:
-   NUMAKER_PFM_NUC472 w/ and w/o XRAM
-   NUMAKER_PFM_M453
-   NUMAKER_PFM_M487/NUMAKER_IOT_M487
-   NUMAKER_IOT_M263A
-   NUMAKER_M252KG

Related targets:
-   NU_PFM_M2351_*
-   NUMAKER_IOT_M263A

Re-implement __PC() by replacing BSP assembly with toolchain built-in.