xbps-src: trying to fix FFLAGS construction

This still isn't perfect. When the common/xbps-src/shutils/chroot.sh
function chroot_init() is called, the value for $XBPS_FFLAGS, which is
defined in common/build-profiles/bootstrap.sh, is empty.

Put the immediate value into the generated /etc/xbps/xbps-src.conf
file until someone finds out where passing the value of $XBPS_FFLAGS
throughout the scripts is missing.

common/cross-profiles/x86_64-musl.sh

@@ -4,3 +4,4 @@ XBPS_TARGET_MACHINE="x86_64-musl"
 XBPS_CROSS_TRIPLET="x86_64-linux-musl"
 XBPS_CROSS_CFLAGS="-mtune=generic"
 XBPS_CROSS_CXXFLAGS="$XBPS_CROSS_CFLAGS"
+XBPS_CROSS_FFLAGS=""