toolchain: update to gcc-7.3.0 and enable retpolines on x86_64 and i586

Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

src/patches/glibc/glibc-2.25-gcc-7.patch

@@ -0,0 +1,23 @@
+diff --git a/malloc/hooks.c b/malloc/hooks.c
+index 4398c0a017..2c6cebc889 100644
+--- a/malloc/hooks.c
++++ b/malloc/hooks.c
+@@ -347,11 +347,18 @@ realloc_check (void *oldmem, size_t bytes, const void *caller)
+       newmem = _int_realloc (&main_arena, oldp, oldsize, nb);
+     }
+ 
++  DIAG_PUSH_NEEDS_COMMENT;
++#if __GNUC_PREREQ (7, 0)
++  /* GCC 7 warns about magic_p may be used uninitialized.  But we never
++     reach here if magic_p is uninitialized.  */
++  DIAG_IGNORE_NEEDS_COMMENT (7, "-Wmaybe-uninitialized");
++#endif
+   /* mem2chunk_check changed the magic byte in the old chunk.
+      If newmem is NULL, then the old chunk will still be used though,
+      so we need to invert that change here.  */
+   if (newmem == NULL)
+     *magic_p ^= 0xFF;
++  DIAG_POP_NEEDS_COMMENT;
+ 
+   __libc_lock_unlock (main_arena.mutex);
+