OpenBLAS

Fix C compiler handling and BINARY=32 mode in CMAKE builds (#2248)

* Fix compiler identification and option setting

* Handle BINARY=32 option on X86_64

* Add xGEMM3M unroll parameters for crossbuild-target CORE2

* Replace bogus mingw64/32bit CI job with actual 32bit build

mingw64 is not multilib-capable, so using an x86_64-mingw with BINARY=32 in the CI was not going to work anyway (but build passed while BINARY=32 was ignored).