machine/stm32f4: refactor common code and add new build tag stm32f4

[ardnew]

As requested to make smaller PRs, this is a subset of changes originally proposed in PR #1322.

Specifically, this change creates a new build tag stm32f4, adds that tag to the currently one board that supports it (stm32f4disco), and refactored code in stm32f4disco to a set of common source files usable by stm32f4disco and any future boards with this same chip.

[sago35]

PR Thank you.
Overall it's very good.
I had no problems with flash on f4disco either.

Please change the review comment mentioned above.

$ tinygo flash --target stm32f4disco -size short ./src/examples/blinky1
   code    data     bss |   flash     ram
   3740       4    4276 |    3744    4280
Open On-Chip Debugger 0.10.0+dev-g5c9350d2 (2020-08-07-12:56)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
WARNING: interface/stlink-v2.cfg is deprecated, please switch to interface/stlink.cfg
Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
Info : clock speed 2000 kHz
Info : STLINK V2J25M14 (API v2) VID:PID 0483:374B
Info : Target voltage: 2.906762
Info : stm32f4x.cpu: hardware has 6 breakpoints, 4 watchpoints
Info : starting gdb server for stm32f4x.cpu on 3333
Info : Listening on port 3333 for gdb connections
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
target halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x08000748 msp: 0x20001000
Info : Unable to match requested speed 8000 kHz, using 4000 kHz
Info : Unable to match requested speed 8000 kHz, using 4000 kHz
** Programming Started **
Info : device id = 0x10076413
Info : flash size = 1024 kbytes
** Programming Finished **
** Resetting Target **
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
Info : Unable to match requested speed 2000 kHz, using 1800 kHz
shutdown command invoked

[sago35]

CI should be fine the next time you push.

[deadprogram]

All tests are passing, so @ardnew is you can please just make the header change requested by @sago35 this PR seems good to go.

[deadprogram]

Squashed/merged this PR, thank you very much for this contribution @ardnew