stm32c5a3nucleo: use a local linker script, register the board

The DFP linker script marks sections (READONLY), which ld.lld rejects
("symbol not found: READONLY"), so every arm-clang link failed. Copy it
to hw/bsp/stm32c5/linker the way stm32c542nucleo does: READONLY dropped
and a 4 KB stack instead of the template's 1 KB.

Add the board's family.json row and CMake presets, and indent the
stm32c5 board.mk flags with two spaces.