gcc6: update template to workaround file-prefix-map and removed libssp package

srcpkgs/gcc6/patches/090-musl.patch

@@ -0,0 +1,11 @@
+--- a/gcc/config/i386/i386.c	2023-04-03 00:23:52.367607124 +0200
++++ -	2023-04-03 00:30:12.723657534 +0200
+@@ -8747,7 +8747,7 @@
+ 			 HOST_WIDE_INT words)
+ {
+   int res = 0;
+-  bool error_p = NULL;
++  bool error_p = false;
+ 
+   if (TARGET_IAMCU)
+     {

srcpkgs/gcc6/template

@@ -70,6 +70,9 @@ post_extract() {
 pre_configure() {
 	# _FORTIFY_SOURCE needs an optimization level.
 	sed -i "/ac_cpp=/s/\$CPPFLAGS/\$CPPFLAGS -O2/" {gcc,libiberty}/configure
+	if [ "$XBPS_TARGET_LIBC" = musl ]; then
+		patch -p1 -i ${FILESDIR}/303-gcc-6.1-musl-libssp.patch
+	fi
 
 	# old isl normally tries to call aclocal-1.14 to reconfigure itself
 	pushd ${wrksrc}/isl
@@ -108,6 +111,10 @@ do_configure() {
 	export CFLAGS="${CFLAGS/-D_FORTIFY_SOURCE=2/}"
 	export CXXFLAGS="${CXXFLAGS/-D_FORTIFY_SOURCE=2/}"
 
+	# file-prefix-map isn't supported in gcc6
+	export CFLAGS="${CFLAGS/-ffile-prefix-map=$wrksrc=./}"
+	export CXXFLAGS="${CXXFLAGS/-ffile-prefix-map=$wrksrc=./}"
+
 	# Disable explicit -fno-PIE, gcc will figure this out itself.
 	export CFLAGS="${CFLAGS//-fno-PIE/}"
 	export CXXFLAGS="${CXXFLAGS//-fno-PIE/}"