Add --enable-debug flag to musl CI build script

Building for x86_64-unknown-linux-musl currently results in an executable lacking debug information for musl libc itself. If you request a backtrace in GDB while control flow is within musl – including sycalls made by musl – the result looks like:

#0  0x0000000000434b46 in __cp_end ()
#1  0x0000000000432dbd in __syscall_cp_c ()
rust-lang#2  0x0000000000000000 in ?? ()

i.e. not very helpful. Adding --enable-debug resolves this, and --enable-optimize re-enables optimisations which default to off given the previous flag.

src/ci/docker/scripts/musl.sh

@@ -40,7 +40,7 @@ if [ ! -d $MUSL ]; then
 fi
 
 cd $MUSL
-./configure --disable-shared --prefix=/musl-$TAG $@
+./configure --enable-optimize --enable-debug --disable-shared --prefix=/musl-$TAG $@
 if [ "$TAG" = "i586" -o "$TAG" = "i686" ]; then
   hide_output make -j$(nproc) AR=ar RANLIB=ranlib
 else