Unify some STM32L4 linker scripts into one

[multiplemonomials]

Summary of changes

Due to #439 , I figured it would help to update the STM32L4 linker scripts to support memory banks. This PR does that and also combines all the copy-pasted linker scripts into a single script.

I didn't want to dive too deeply into STM32L4 memory configurations at present b/c IDK if I have the mental bandwidth, so I didn't go full potato on unifying all the linker scripts and system clock files (yet). Just wanted to clean up these few duplicates and add memory bank support.

Impact of changes

• All STM32L4 targets with two SRAM banks (where IRAM1 is the larger bank and IRAM2 is the smaller bank with the stack) now use a single linker script.
• All of the above targets now support split heap (most did, but a few didn't for some reason)
• All of the above targets now support memory bank configuration in the linker script

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] 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

Ran the heap tests on my NUCLEO_L452RE_P board, and everything seemed to run properly.