make gcc wrapper support -shared correctly

it was previously attempting to link start files as part of shared
objects. this is definitely wrong and depending on the platform and
linker could range from just adding extraneous junk to introducing
textrels to making linking fail entirely.

tools/gen-musl-gcc.sh

@@ -29,6 +29,7 @@ for i ; do
 case "$skip$i" in
 -I|-L) skip=--- ; continue ;;
 -[cSE]|-M*) nolink=1 ;;
+-shared) nocrt=1 ;;
 -*) ;;
 *) havefile=1 ;;
 esac
@@ -37,13 +38,17 @@ done
 
 [ "$havefile" ] || nolink=1
 
+[ "$nolink" ] && nocrt=1
+
+[ "$nocrt" ] || set -- "$libc_start" "$libc_crt" "$@" "$libc_end" \
+
 [ "$nolink" ] || {
 tmp_specs=$HOME/.specs.tmp.$$
 printf '*link_libgcc:\n\n\n' > "$tmp_specs" || exit 1
 exec 3<"$tmp_specs"
 rm -f "$tmp_specs"
-set -- -specs=/proc/self/fd/3 "$libc_start" "$libc_crt" "$@" "$libc_end" \
-  -Wl,--start-group -lc -lgcc -lgcc_eh -Wl,--end-group \
+set -- -specs=/proc/self/fd/3 "$@" \
+  -Wl,--as-needed -Wl,--start-group -lc -lgcc -lgcc_eh -Wl,--end-group \
   -Wl,-dynamic-linker,"$ldso_pathname" -Wl,-nostdlib
 }