stm32: Disable computed goto on constrained boards.

[jimmo]

Saves ~1kiB. Add comment to this effect to mpconfig.h.

I saw CI failures on a PR for L072 due to code size, so this gets a bit of extra headroom.

See #7680 (comment) for some recent benchmarks for computed goto (on M4).

[codecov-commenter]

Codecov Report

Merging #7685 (c3ac402) into master (44818d1) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7685   +/-   ##
=======================================
  Coverage   98.25%   98.26%           
=======================================
  Files         154      154           
  Lines       20071    20071           
=======================================
+ Hits        19721    19722    +1     
+ Misses        350      349    -1     

Impacted Files	Coverage Δ
py/runtime.c	99.24% <0.00%> (-0.16%)	⬇️
py/obj.c	97.61% <0.00%> (ø)
py/objfun.c	100.00% <0.00%> (ø)
py/lexer.c	98.99% <0.00%> (+<0.01%)	⬆️
py/parse.c	99.17% <0.00%> (+0.20%)	⬆️
py/bc.c	89.69% <0.00%> (+1.03%)	⬆️

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[dpgeorge]

Build failures on stm32 need to be fixed (it's obvious why they fail).

[jimmo]

Build failures on stm32 need to be fixed (it's obvious why they fail).

Done.

[dpgeorge]

Merged in b51e7e9