[gdb/testsuite] Simplify regexp in gdb.rocm/watchpoint-at-end-of-shader.exp

In gdb.rocm/watchpoint-at-end-of-shader.exp, we have:
...
	    [multi_line "Switching to \[^\r\n\]+(?=\r\n)" \
		"" \
...

This expands to "Switching to \[^\r\n\]+(?=\r\n)\r\n".

The lookahead '(?=\r\n)' is superfluous (because it's followed by '\r\n'), so
drop it.

While we're at it, reduce escaping by using {}.