Allow VECT_TAB_OFFSET to be overridden externally (IDE or Makefile)

[theadib]

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This topic is longstanding and has been discussed but not solved so far: https://github.com/STMicroelectronics/STM32CubeF4/issues/65

This small thing was needed, however I wish you can revert the need for USER_VECT_TAB_ADDRESS because this is absolutely pointless.

Regards. Adib.

[ALABSTM]

ST Internal Reference: 116475