Don't use TARGET unless the target options are *not* specified

This eliminates the possibility of inlining target failures for ondemand

Also makes it so we always compile common architectures needed by simdjson.cpp in simdjson.h, since amalgamation has no way to reason about whether to include / exclude it.

src/implementation.cpp

@@ -155,6 +155,7 @@ simdjson_warn_unused bool validate_utf8(const char *buf, size_t len) noexcept {
 
 const implementation * builtin_implementation() {
   static const implementation * builtin_impl = available_implementations[STRINGIFY(SIMDJSON_BUILTIN_IMPLEMENTATION)];
+  assert(builtin_impl);
   return builtin_impl;
 }