Quiet ptrace error ESRCH in regsets_fetch_inferior_registers
    
    When I run process-dies-while-detaching.exp with GDBserver, I see many
    warnings printed by GDBserver,
    
    ptrace(regsets_fetch_inferior_registers) PID=26183: No such process
    ptrace(regsets_fetch_inferior_registers) PID=26183: No such process
    ptrace(regsets_fetch_inferior_registers) PID=26184: No such process
    ptrace(regsets_fetch_inferior_registers) PID=26184: No such process
    
    regsets_fetch_inferior_registers is called when GDBserver resumes each
    lwp.
    
     #2  0x0000000000428260 in regsets_fetch_inferior_registers (regsets_info=0x4690d0 <aarch64_regsets_info>, regcache=0x31832020)
        at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:5412
     #3  0x00000000004070e8 in get_thread_regcache (thread=0x31832940, fetch=fetch@entry=1) at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/regcache.c:58
     #4  0x0000000000429c40 in linux_resume_one_lwp_throw (info=<optimized out>, signal=0, step=0, lwp=0x31832830)
        at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:4463
     #5  linux_resume_one_lwp (lwp=0x31832830, step=<optimized out>, signal=<optimized out>, info=<optimized out>)
        at /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:4573
    
    The is the case that threads are disappeared when GDB/GDBserver resumes
    them.  We check errno for ESRCH, and don't print error messages, like
    what we are doing in regsets_store_inferior_registers.
    
    gdb/gdbserver:
    
    2016-08-04  Yao Qi  <yao.qi@linaro.org>
    
    	* linux-low.c (regsets_fetch_inferior_registers): Check
    	errno is ESRCH or not.