doc: sandbox: Add a note of disabling LTO when using gdb

Image created by LTO is not friendly to debugger, let's document this.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

Mention CONFIG_CC_OPTIMIZE_FOR_DEBUG and LLDB.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>