testsuite: Fix gcc.target/arm/multilib.exp use of gcc_opts

This patch fixes an incorrect parameter passing for $gcc_opts, which
produces a DejaGnu error: (DejaGnu) proc "gcc_opts" does not exist.

2020-08-12  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* gcc.target/arm/multilib.exp: Fix parameter passing for gcc_opts.