add stackprotector support on musl32

In order to support stackprotector on musl32, this change import a
couple of patches from alpinelinux:
1. libssp_nonshared.a is built alongside musl's libc
2. the above library is automatically linked when compiling with gcc6
   or gcc7

pkgs/development/compilers/gcc/7/default.nix

@@ -72,6 +72,10 @@ let majorVersion = "7";
       })
       ++ optional langFortran ../gfortran-driving.patch
       ++ optional (targetPlatform.libc == "musl" && targetPlatform.isPower) ../ppc-musl.patch
+      ++ optional (targetPlatform.libc == "musl" && targetPlatform.isx86_32) (fetchpatch {
+        url = "https://git.alpinelinux.org/aports/plain/main/gcc/gcc-6.1-musl-libssp.patch";
+        sha256 = "1jf1ciz4gr49lwyh8knfhw6l5gvfkwzjy90m7qiwkcbsf4a3fqn2";
+      })
       ++ optional (targetPlatform.libc == "musl") ../libgomp-dont-force-initial-exec.patch
 
       # Obtain latest patch with ../update-mcfgthread-patches.sh