semihosting::has_resumable_exit field

When SEMIHOSTING_SYS_EXIT is called outside a debug session, things are simple, the openocd process calls exit() and passes the value returned by the target. When SEMIHOSTING_SYS_EXIT is called during a debug session, by default execution returns to the debugger, leaving the debugger in a HALT state, similar to the state entered when encountering a break. In some use cases, it is useful to have SEMIHOSTING_SYS_EXIT return normally, as any semihosting call, and do not break to the debugger. The standard allows this to happen, but the condition to trigger it is a bit obscure ("by performing an RDI_Execute request or equivalent"). To make the SEMIHOSTING_SYS_EXIT call return normally, enable this variable via the dedicated command (default: disabled).