GDB: testsuite: x86, amd64, i386: Don't return value from top-level (manual)

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

In amd64-shadow-stack-cmds.exp the return statements have extra
indentation due to being inside a save_vars block.

In i386-bp_permanent.exp this was caused by misaligned lines, which are
now fixed.

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