#3630 UB in the compiler

[Philipp Klaus Krause]

Having compiled the assembler and linker with CC="gcc -fsanitize-address-use-after-scope -fsanitize=undefined" CXX="g++ -fsanitize-address-use-after-scope -fsanitize=undefined" I saw many cases of code that has UB. I'm trying to fix most of them by a commit later today (assuming regression tests pass), but there are a few I haven't fixed yet. The ones most often hit are:
* Overflows in SDCCval.c
* Out-of-bounds array accesses in the ds390 port.
* An issue about reading an uninitialized bool in SDCCralloc.hpp (this one is most likely a false positive, IMO, we're just copying a partially-initialized struct, and not really using that bool).

[Philipp Klaus Krause]

• I moved the mos6502-specific part into its own ticket: [bugs:#3631].
• I fixed the ds390-, mcs51-, pdk- and hc08-specific parts that are triggered when compiling the standard library in [r14260].
• There is still some UB in src/SDCCval.c and src/SDCCicode.c (mostly signed integer overflow, and that one bool in src/SDCCralloc.hpp).
• Looking into further UB triggered when compiling the regression tests, I found an issue for z80, which is fixed in [r14261].