Regression test framework: add a possibility to define different (additional) SDCC command line options on per test/target basis.
The simplest way is to define test/target specific SDCCFLAGS, for example:
SDCCFLAGS_<target>_<test> = ...
where <target> is target subdirectory name and <test> is the test file name:
SDCCFLAGS_ds390_bug-1646602 = --funsigned-char
All such special SDCCFLAGS could be defined in a separate file, for example support/regression/sdccflags.mk, which would be included in support/regression/Makefile.