Mark register unavailable when PTRACE_PEEKUSER fails

Mark register unavailable when PTRACE_PEEKUSER fails

As described in PR 18749, GDB/GDBserver may get an error on accessing
memory or register because the thread may disappear.  However, some
path doesn't expect the error.  This patch fixes this problem by
marking the register unavailable when PTRACE_PEEKUSER fails instead
of throwing error.

gdb/gdbserver:

2018-01-16  Yao Qi  <yao.qi@linaro.org>

	PR gdb/18749
	* linux-low.c (fetch_register): Call supply_register instead of
	error.