Modify scatter files to add heap load region, and remove hard coded
values. Add supported c libs and bare metal profile to `targets.json`
configurations. Affects the following targets: EFM32GG_STK3700,
TB_SENSE_12, and EFM32GG11_STK3701.

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

--legacyalign, --no_legacyalign are deprecated from ARMC6 compiler, in order to
remove deprecated flags all linker files (GCC and IAR as well to have uniformity)
should strictly align to 8-byte boundary

* Make memory sections configurable in linker files
* Dynamically determine vector location in flash for NVIC relocation
* Advertise bootloader support in targets.json

Now that we have targets.json, we get target inheritance and can use it to clean up the EFM32 folder structure.
* In the top-level EFM32 folder, there are now folders per MCU family (Giant, Leopard, ...)
* Those family folders contain the CMSIS headers in the 'device' subfolder, as well as global family headers (i.e. mapping of pins to peripherals)
* Inside of the family folder, there is a per-target folder containing target settings. In the future, we'll want to get rid of those by using the config system provided by targets.json