Linking `libomp.so` on 32-bit SPARC `FAIL`s with
```
ld: fatal: file projects/openmp/runtime/src/CMakeFiles/omp.dir/z_Linux_asm.S.o: wrong ELF class: ELFCLASS64
```
This was a 1-stage build with a 64-bit-default `gcc`. Unlike the C++
sources, which were compiled as 32-bit objects due to the use of
`-DCMAKE_CXX_FLAGS=-m32`, the assembler sources were not.
This patch simplifies passing `-m32`: instead of doing it per
architecture, `-m32` is now always passed when the target uses 32-bit
pointers and supports the option.
Tested on `sparc-sun-solaris2.11`, `sparcv9-sun-solaris2.11`,
`sparc-unknown-linux-gnu`, `sparc64-unknown-linux-gnu`,
`i386-pc-solaris2.11`, `amd64-pc-solaris2.11`, `i686-pc-linux-gnu`, and
`x86_64-pc-linux-gnu`.
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