Add i586 (pre-P6) support, shell script cleanups

[thp]

Add support for building a i586-w64-mingw32 toolchain, which creates binaries without the P6-only cmov instructions (among other things). This should allow generation of binaries for really really old CPUs (pre-Pentium Pro, pre-K7/Athlon).

I have tested building a small example application using the toolchain and it seems to work fine with wine under Linux. Also, verified with i586-w64-mingw32-objdump -d ... | grep cmov that it indeed doesn't generate any cmov instructions where the i686-w64-mingw32 toolchain does generate cmov instructions (have yet to test it on an old CPU).

As part of this addition, changed some shell script parts and fixed some shellcheck issues.