gdb/testsuite: fix running gdb.python/py-explore-cc with clang

The test gdb.python/py-explore-cc.exp was showing one unexpected
failure. This was due to how clang mapped instructions to lines,
resulting in the inferior seemingly stopping at a different location.

This patch adds a nop line in the relevant location so we don't need to
add XFAILs for existing clang releases, if this gets solved in future
versions.

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