[gdb/ax] small "setv" fix and documentation's adjustment.
    
    gdb/doc/agentexpr.texi documents the "setv" opcode as follow:
    
        @item @code{setv} (0x2d) @var{n}: @result{} @var{v}
        Set trace state variable number @var{n} to the value found on the top
        of the stack.  The stack is unchanged, so that the value is readily
        available if the assignment is part of a larger expression.  The
        handling of @var{n} is as described for @code{getv}.
    
    The @item line is incorrect (and does not match with its
    description), so this patch fixes it.
    
    Additionally, in gdb/common/ax.def we find the line:
    
        DEFOP (setv, 2, 0, 0, 1, 0x2d)
    
    From the comment earlier in the file:
    
           Each line is of the form:
    
           DEFOP (name, size, data_size, consumed, produced, opcode)
    [...]
           CONSUMED is the number of stack elements consumed.
           PRODUCED is the number of stack elements produced.
    
    which is saying that nothing is consumed and one item is produced.
    Both should be 0 or both should be 1.
    
    This patch sets them both to 1, which seems better since if nothing
    is on the stack an error will occur.
    
    gdb/ChangeLog:
    
            * common/ax.def (setv): Fix consumed entry in setv DEFOP.
    
    gdb/doc/ChangeLog:
    
            * agentexpr.texi (Bytecode Descriptions): Fix summary line for setv.
    
    Tested on x86_64-linux.