Dmod_SyncCode: make a loaded module's code safe to execute

[JohnAmadis]

Summary

Adds the SAL function Dmod_SyncCode(Address, Size). The module loader calls it once a module is loaded and relocated, before any of its code runs.

• Why it is needed: on a core with caches, the freshly written code may still sit in the data cache. The instruction cache may also hold an unloaded module's code from the same addresses.
• What the system does: it cleans the data cache for the range and invalidates the instruction cache. dmod-boot implements it for the Cortex-M7 in choco-technologies/dmod-boot feature/cortex-m7-caches.
• Default: a weak implementation that does nothing, right for cores without caches and for hosts.
• Loader change: Dmod_Ldr_Load() calls Dmod_SyncCode(Context->Data, Context->Size) after loading the BSS and before reading the required modules.

Testing

On an STM32F746G-DISCO with I- and D-cache enabled and the SDRAM cached write-back:

• every module loaded at boot starts and runs: drivers, systemd services, dmview, the dmell console (some of them load into the SDRAM, where the I-cache now matters);
• the dmview demo runs for many minutes.

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