common: remove rdimon when debug is enabled

[tlyu]

Save about 1.2KiB (depending on GCC version) by redirecting _write directly in libnosys, instead of pulling in librdimon.

Detailed description

• Improves firmware footprint on native BMP when debug output is enabled
• There doesn't appear to be a current need to use librdimon semihosting capabilities
• libnosys I/O functions such as _write can be retargeted directly without using semihosting
• Initializing librdimon semihosting brings in a lot of unused code

I haven't tested on other probe platforms, so I might have unintentionally broken something.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

N/A

[ALTracer]

Please rebase on main -- half of usb_serial.c moved to syscalls.c. I just tried rebasing your commit on main and got a -1344 byte size-diff for meson setup build-stlink --cross-file=cross-file/stlink.ini -Ddebug_output=true -Dbmd_bootloader=false -Dtargets="cortexm,at32f4,rp,stm", which is very nice. While writes definitely don't have to go through DebugMon handler, I expect another bonus: RDP1-locked stlinkv2 (and RDP2-locked stlinkv3) may stop hanging on their first DEBUG_INFO() call, because it no longer uses breakpoints or any (disabled) debug facilities. I can't confirm whether ST disables DebugMon, but it might be the case. Needs testing on ST_BOOTLOADER=1 stlinkv2 and stlinkv3 to confirm the benefit (of allowing users to do all the DEBUG_INFO prints on-probe without BMDA). Or any stm32f103 with normal BMD bootloader but read protection activated after confirming DEBUG_INFO works, to observe that it starts crashing.
If you don't want to handle all the Makefile.inc per-platform, I might handle it myself, checking the result on a few bluepills. f072 can't use debugmon anyways (goes into hardfault), so it's a win-win for it.

 src/platforms/common/stm32/meson.build |  2 +-
 src/platforms/common/syscalls.c        | 65 -----------------------------------------------------------------
 src/platforms/common/usb_serial.c      | 12 ------------
 3 files changed, 1 insertion(+), 78 deletions(-)

[tlyu]

Thanks for taking a look! I will try to make time to work on this a bit next week.