toolchain: Add GCC 9.1.0 release

Most of the patches are copied over from GCC 8.3.

The following patches are backported from the GCC 9.X development branch:
toolchain/gcc/patches/9.1.0/970-recompute-dom-fast-queries-before-vn.patch
toolchain/gcc/patches/9.1.0/975-g++-ICE-with-generic-lambda.patch

The specs file changed with gcc 9, now it contains "%@{L*}" instead of
"%{L*}" in older GCC versions.

Signed-off-by: Joseph Benden <joe@benden.us>

toolchain/gcc/patches/9.1.0/920-specs_nonfatal_getenv.patch

@@ -0,0 +1,22 @@
+Author: Jo-Philipp Wich <jow@openwrt.org>
+Date:   Sat Apr 21 03:02:39 2012 +0000
+
+    gcc: add patch to make the getenv() spec function nonfatal if requested environment variable is unset
+    
+    SVN-Revision: 31390
+
+--- a/gcc/gcc.c
++++ b/gcc/gcc.c
+@@ -9318,8 +9318,10 @@ getenv_spec_function (int argc, const ch
+     }
+ 
+   if (!value)
+-    fatal_error (input_location,
+-		 "environment variable %qs not defined", varname);
++    {
++      warning (input_location, "environment variable %qs not defined", varname);
++      value = "";
++    }
+ 
+   /* We have to escape every character of the environment variable so
+      they are not interpreted as active spec characters.  A