x86/x64: Call external symbols directly from interpreter code.

Except for ELF/x86 PIC, where it's easier to use wrappers.

src/host/buildvm.h

@@ -82,6 +82,7 @@ typedef struct BuildCtx {
   const char *beginsym;
   /* Strings generated by DynASM. */
   const char *const *globnames;
+  const char *const *extnames;
   const char *dasm_ident;
   const char *dasm_arch;
   /* Relocations. */