Fix getrbp()

Fix getrbp()

The following compiler warning is emitted for the BACKTRACE_X86STACK
part of lib/isc/backtrace.c:

    backtrace.c: In function ‘getrbp’:
    backtrace.c:142:1: warning: no return statement in function returning non-void [-Wreturn-type]

While getrbp() stores the value of the RBP register in the RAX register
and thus does attempt to return a value, this is not enough for an
optimizing compiler to always produce the expected result.  With -O2,
the following machine code may be generated in isc_backtrace_gettrace():

    0x00007ffff7b0ff7a <+10>:	mov    %rbp,%rax
    0x00007ffff7b0ff7d <+13>:	mov    $0x17,%eax
    0x00007ffff7b0ff82 <+18>:	retq

The above is equivalent to:

    sp = (void **)getrbp();
    return (ISC_R_NOTFOUND);

and results in the backtrace never getting printed.

Fix by using an intermediate variable.  With this change in place, the
machine code generated with -O2 becomes something like:

    0x00007ffff7af5638 <+24>:	mov    $0x17,%eax
    0x00007ffff7af563d <+29>:	mov    %rbp,%rdx
    0x00007ffff7af5640 <+32>:	test   %rdx,%rdx
    0x00007ffff7af5643 <+35>:	je     0x7ffff7af56bd <isc_backtrace_gettrace+157>
    ...
    0x00007ffff7af56bd <+157>:	retq

(Note that this method of grabbing a stack trace is finicky anyway
because in order for RBP to be relied upon, -fno-omit-stack-frame must
be present among CFLAGS.)