* lib/target.exp (default_target_compile): Prepend multilib_flags,

	* lib/target.exp (default_target_compile): Prepend multilib_flags,
	instead of appending it.

Some GCC testcases need explicit GCC options to properly run. For
example gcc.target/i386/pr32219-1.c has -fpie specified explicitly:

/* { dg-options "-O2 -fpie" } */

But with multlib, eg:
make check-gcc RUNTESTFLAGS="--target_board='unix{-fpic}'"

-fpic is appended to the command line options, which overrides 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
updates default_target_compile to prepend multilib_flags, instead of
appending it.

Signed-off-by: Ben Elliston <bje@gnu.org>