GDB: testsuite: riscv: Don't return value from top-level (manual)

This patch manually changes the "return -1" and "return 0" statements
that weren't caught by the sed command.

This happened only in one testcase due to the statements being inside a
foreach_with_prefix block, and in the others because the return
statement was in a misaligned line, which is now fixed.

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