[OpenMP] Don't use libproc on Solaris (#142379)
`openmp` currently doesn't compile on 32-bit Solaris:
```
FAILED: projects/openmp/runtime/src/CMakeFiles/omp.dir/z_Linux_util.cpp.o
[...]
In file included from openmp/runtime/src/z_Linux_util.cpp:78:
In file included from /usr/include/libproc.h:25:
In file included from /usr/include/gelf.h:10:
/usr/include/libelf.h:22:2: error: "large files are not supported by libelf"
22 | #error "large files are not supported by libelf"
| ^
In file included from openmp/runtime/src/z_Linux_util.cpp:78:
/usr/include/libproc.h:42:2: error: "Cannot use libproc in the large file compilation environment"
42 | #error "Cannot use libproc in the large file compilation environment"
| ^
```
Looking closer, there's no point in using `Pgrab` (the only interface
from `<libproc.h>`) at all: the resulting `ps_prochandle_t *` isn't used
in the remainder of the code and the original PR #82930 gives no
indication why it is deemed necessary/useful.
While at it, this patch switches to use `/proc/self/xmap`, matching
`compiler-rt`'s `sanitizer_procmaps_solaris.cpp`, and makes some minor
formatting fixes.
Tested on `sparc-sun-solaris2.11`, `sparcv9-sun-solaris2.11`,
`i386-pc-solaris2.11`, and `amd64-pc-solaris2.11`.
