arc: Don't use multiline in arc-disassembler-options.exp test

Breaking a TCL string to several lines leads to adding of extra
symbols to the resulting expect string. In turn, this leads to
failing of all test cases in gdb.arch/arc-disassembler-options.exp
testsuite. It's necessary to use multi_line function in such
cases.

Approved-By: Tom Tromey <tom@tromey.com>