btrace: preserve call stack on function switch

btrace: preserve call stack on function switch

On 64-bit FC25, the _dl_runtime_resolve function uses a conditional branch to
'call' a particular variant optimized for that system:

    (gdb) disas _dl_runtime_resolve_avx_opt
    Dump of assembler code for function _dl_runtime_resolve_avx_opt:
       0x00007ffff7deeb60 <+0>: push   %rax
       0x00007ffff7deeb61 <+1>: push   %rcx
       0x00007ffff7deeb62 <+2>: push   %rdx
       0x00007ffff7deeb63 <+3>: mov    $0x1,%ecx
       0x00007ffff7deeb68 <+8>: xgetbv
       0x00007ffff7deeb6b <+11>: mov    %eax,%r11d
       0x00007ffff7deeb6e <+14>: pop    %rdx
       0x00007ffff7deeb6f <+15>: pop    %rcx
       0x00007ffff7deeb70 <+16>: pop    %rax
       0x00007ffff7deeb71 <+17>: and    $0x4,%r11d
       0x00007ffff7deeb75 <+21>: bnd je 0x7ffff7def4a0 <_dl_runtime_resolve_sse_vex>
    End of assembler dump.

When computing the function-level trace, btrace treats this as a switch from
_dl_runtime_resolve_avx_opt to _dl_runtime_resolve_sse_vex.  We know that we
switched functions but we can't really say in which caller/callee relationship
those two functions are.

In addition to preserving the indentaion level, also preserve the caller
information.  This is a heuristic since we don't really know.  But at least in
this case, this seems to be the right thing to do.

This fixes a fail in gdb.btrace/rn-dl-bind.exp on 64-bit FC25.

gdb/
	* btrace.c (ftrace_new_switch): Preserve up link and flags.