[CMake] Add support for building on illumos (#74930)

illumos has an older version of the Solaris linker that does not
support the GNU version script compat nor version scripts and does
not support -Bsymbolic-functions. Treat illumos linker separately.

The libclang/CMakeLists part lifted from NetBSD's pkgsrc.

Build tested on Solaris 11.4 and OpenIndiana 2023.10.

/usr/bin/ld --version

ld: Software Generation Utilities - Solaris Link Editors: 5.11-1.3260

ld: Software Generation Utilities - Solaris Link Editors: 5.11-1.1790 (illumos)