Fix source and update sha256 for gmp 6.3.0

Author: GuillaumeGomez

files/rustc/gcc.toml

@@ -1,5 +1,5 @@
 [[files]]
 name = "rustc/gcc/gmp-6.3.0.tar.bz2"
-source = "https://gmplib.org/download/gmp/gmp-6.3.0.tar.xz"
-sha256 = "a3c2b80201b89e68616f4ad30bc66aee4927c3ce50e33929ca819d5c43538898"
+source = "https://gmplib.org/download/gmp/gmp-6.3.0.tar.bz2"
+sha256 = "ac28211a7cfb609bae2e2c8d6058d66c8fe96434f740cf6fe2e47b000d1c20cb"
 license = "GNU General Public License"