gcc{5,6,7,8}: share libgomp patch, propagate musl config flags

pkgs/development/compilers/gcc/6/default.nix

@@ -61,6 +61,7 @@ let version = "6.4.0";
       ++ optional langFortran ../gfortran-driving.patch
       ++ [ ../struct-ucontext.patch ../struct-sigaltstack.patch ] # glibc-2.26
       ++ optional langJava [ ../struct-ucontext-libjava.patch ] # glibc-2.26
+      ++ optional (targetPlatform.libc == "musl") ../libgomp-dont-force-initial-exec.patch
       ;
 
     javaEcj = fetchurl {
@@ -336,7 +337,12 @@ stdenv.mkDerivation ({
       # On Illumos/Solaris GNU as is preferred
       "--with-gnu-as" "--without-gnu-ld"
     ]
-    ++ optional (targetPlatform == hostPlatform && targetPlatform.libc == "musl") "--disable-libsanitizer"
+    ++ optionals (targetPlatform == hostPlatform && targetPlatform.libc == "musl") [
+      "--disable-libsanitizer"
+      "--disable-symvers"
+      "libat_cv_have_ifunc=no"
+      "--disable-gnu-indirect-function"
+    ]
   ;
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;