Convert ARM assembler startup files to GCC-compatible during build

[bjc]

As I understand it, the board startup code in https://github.com/CommunityGD32Cores/ArduinoCore-GD32/tree/main/system/CMSIS/GD/GD32F30x/Source/GCC is being generated by hand run sed scripts from the upstream firmware library source in GD32F30x_Firmware_Library_V2.0.2/Firmware/CMSIS/GD/GD32F30x/Source/ARM (which is missing in this repository).

In order to better support drop-in firmware library upgrades, the GCC-compatible startup code should be generated as part of the build process, with the relevant translation scripts checked in to the source tree.