multilib_flags is placed with the wrong order

Some testcases need explicit GCC options to properly run, like
gcc.target/i386/sse2-init-v2di-2.c has

/* { dg-options "-O2 -msse4 -march=core2 -dp" } */

-march=core2 is specified explicitly.  But with multlib, like

make check-gcc RUNTESTFLAGS="--target_board='unix{-march=k8}'

-march=k8 is appended to the command line options, which overrides
the the command line options specified by dg-options.  multlib flags
should be placed at the beginning of the command line options, not at
the end.  This patch adds gcc_default_target_compile, which is
almost identical to default_target_compile in dejagnu, except that
it has

    if {[board_info $dest exists multilib_flags]} {
        set add_flags "[board_info $dest multilib_flags] $add_flags"
    }

instead of

    if {[board_info $dest exists multilib_flags]} {
        append add_flags " [board_info $dest multilib_flags]"
    }

See:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59971

for details.

-- 
H.J.