split linker out of elf module

split linker out of elf module

* module/Makefile.am:
* module/system/vm/linker.scm: New file, split out of (system vm elf).
  (make-string-table, string-table-intern): Export under their bare
  names, instead of make-elf-string-table and elf-string-table-intern.

* module/system/vm/elf.scm: Remove linking capabilities.
  (string-table-ref): Export.

* module/language/objcode/elf.scm (bytecode->elf): Adapt to use (system
  vm linker).

* test-suite/tests/linker.test: New test.