libs/libc: Generate the system symbol tables in the CMake build - #20489

Summary

With CONFIG_EXECFUNCS_SYSTEM_SYMTAB, the make build generates exec_symtab.c from libc.csv, libm.csv and syscall.csv with the host tool mksymtab (libs/libc/Makefile). CONFIG_LIBC_ELF_SYSTEM_SYMTAB does the same for elf_sys_symtab.c. The CMake build did neither, so a CMake configuration with one of these options did not link:

binfmt/binfmt_execsymtab.c:127: undefined reference to `g_symtab'
binfmt/binfmt_execsymtab.c:127: undefined reference to `g_nsymbols'

This PR does the same in libs/libc/CMakeLists.txt: it builds mksymtab as a host tool with nuttx_build_host_target(), sorts the three lists by name, generates the table with the configured array and count names, and adds it to the C library. mksymtab runs in the build directory with relative names, so the output is the same as from the make build.

This came up in review of #20132, which documents that a CMake configuration needs the table from elsewhere. That note goes once this merges.

Impact

CMake builds with CONFIG_EXECFUNCS_SYSTEM_SYMTAB or CONFIG_LIBC_ELF_SYSTEM_SYMTAB. No change to the make build or to CMake builds without these options.

Testing

Host: macOS arm64, Arm GNU Toolchain 13.2.rel1, CMake with Ninja. mps2-an500:xipfs with CONFIG_ELF, CONFIG_LIBC_EXECFUNCS, CONFIG_EXECFUNCS_HAVE_SYMTAB and CONFIG_EXECFUNCS_SYSTEM_SYMTAB:

mps2-an500:xipfs with CONFIG_LIBC_ELF_HAVE_SYMTAB, CONFIG_LIBC_ELF_SYSTEM_SYMTAB and the names g_elf_symtab/g_elf_nsymbols: links, and nm shows both symbols. The CMake image boots to NSH under QEMU. tools/checkpatch.sh -c -u -m -g passes.