gcc: Temporarily move to https://github.com/keith-packard/gcc

This provides access to the additional commits present in the
riscv-picolibc-oslib branch.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

.gitmodules

@@ -4,7 +4,7 @@
 	branch = binutils-2_39-branch
 [submodule "gcc"]
 	path = gcc
-	url = https://gcc.gnu.org/git/gcc.git
+	url = https://github.com/keith-packard/gcc
 	branch = releases/gcc-12
 [submodule "glibc"]
 	path = glibc