LPC1768: Update linker script, support split heap

[multiplemonomials]

Summary of changes

I ran into an issue when running the new network tests where the LPC1768 was running out of heap space. This MCU has two 32k banks of RAM, where one bank is used as heap, bss, and stack, half of the other bank is used for Ethernet buffers, and the second half of the third bank is unused.

This PR activates split heap support on the device, enabling all unused space to be used as heap. This nearly triples the amount of available heap on this target (if Ethernet is not used at least)! And it can now run the nanostack TCP test without crashing from running out of memory (though it still doesn't pass...).

Impact of changes

• LPC1678 heap space increased to ~48k if Ethernet unused, or ~32k if Ethernet used
• LPC1768 linker script now supports memory bank configuration

Migration actions required

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Test run in progress...