Bug 7558: Complete implementation of the boot-time symbol table

* sources/dfmc/llvm-back-end/llvm-primitives-symbol.dylan
  (%oblist): Define initial value as the empty vector.
  (op--lookup-symbol): Complete implementation of linear search of %oblist.
  (op--as-lowercase): New auxiliary emitter method.
  (primitive primitive-strncasecmp): New auxiliary primitive for
   case-insensitive characater comparisons.
  (op--register-symbol): Complete implementation of adding symbols to %oblist.
  (primitive primitive-resolve-symbol): Complete emitter implementation.
   Remove mapped attribute, and add explicit casts.
  (primitive primitive-string-as-symbol): Complete emitter implementation.
  (primitive primitive-preboot-symbols): Complete emitter implementation.

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($wrapper-classes): Add <symbol> to the list of wrappers referenced
   by the runtime.