driver: run ${target}-as as a last resort

As of now the driver tries to locate `as` (and `ld`) somethere in $PATH
if nothing was found in libsubdir and tooldir. This is a bad idea for
a cross-compiler. Use DEFAULT_TARGET_MACHINE-{as,ld} instead.

Note that `collect2` already behaves that way (which is a good thing).

crossgcc-target-as-ld.patch

@@ -0,0 +1,27 @@
+diff -Nru gcc-8.4.1-20200305.orig/gcc/gcc.c gcc-8.4.1-20200305/gcc/gcc.c
+--- gcc-8.4.1-20200305.orig/gcc/gcc.c	2020-03-10 11:34:46.000000000 +0000
++++ gcc-8.4.1-20200305/gcc/gcc.c	2021-06-11 20:54:35.938336123 +0000
+@@ -2771,6 +2771,7 @@
+ 	     bool do_multi)
+ {
+   struct file_at_path_info info;
++  char *path  = NULL;
+ 
+ #ifdef DEFAULT_ASSEMBLER
+   if (! strcmp (name, "as") && access (DEFAULT_ASSEMBLER, mode) == 0)
+@@ -2798,9 +2799,14 @@
+   info.suffix_len = strlen (info.suffix);
+   info.mode = mode;
+ 
+-  return (char*) for_each_path (pprefix, do_multi,
++  path = (char*) for_each_path (pprefix, do_multi,
+ 				info.name_len + info.suffix_len,
+ 				file_at_path, &info);
++#ifdef CROSS_DIRECTORY_STRUCTURE
++  if (! path && (! strcmp (name, "as") || ! strcmp (name, "ld")))
++      path = concat (DEFAULT_TARGET_MACHINE "-", name, NULL);
++#endif
++  return path;
+ }
+ 
+ /* Ranking of prefixes in the sort list. -B prefixes are put before