[gdb/testsuite] Fix gdb.base/vla-optimized-out.exp for ppc64le

[gdb/testsuite] Fix gdb.base/vla-optimized-out.exp for ppc64le

On powerpc64le-linux, I run into:
...
(gdb) PASS: gdb.base/vla-optimized-out.exp: o1: printed optimized out vla
p sizeof (a)^M
$2 = <optimized out>^M
(gdb) FAIL: gdb.base/vla-optimized-out.exp: o1: \
  printed size of optimized out vla
...

The problem happens as follows.

In order to find the size of the optimized out vla, gdb needs to evaluate:
...
<155> DW_AT_upper_bound : 13 byte block: f3 1 53 23 1 8 20 24 8 20 26 31 1c \
  (DW_OP_GNU_entry_value: (DW_OP_reg3 (r3)); DW_OP_plus_uconst: 1;
   DW_OP_const1u: 32; DW_OP_shl; DW_OP_const1u: 32; DW_OP_shra; DW_OP_lit1;
   DW_OP_minus)
...

When trying to evaluate DW_OP_GNU_entry_value, it looks for a call site
matching the pc, but doesn't find it:
...
$ gdb -q -batch outputs/gdb.base/vla-optimized-out/vla-optimized-out-o1 \
  -ex "break f1" -ex run -ex "set debug entry-values 1" -ex "print sizeof (a)"
Breakpoint 1 at 0x1000067c: file vla-optimized-out.c, line 34.

Breakpoint 1, f1 (i=5) at vla-optimized-out.c:34
34      }
DW_OP_entry_value resolving cannot find DW_TAG_call_site 0x100006b0 in main
$1 = <optimized out>
....

The call site lookup fails because the call site label .LVL4:
...
        bl f1    # 11   *call_value_nonlocal_aixdi      [length = 8]
        nop
.LVL4:
...
is not placed directly after the bl insn.  This is gcc PR target/107909.

However, after manually fixing the .s file we have instead:
...
Cannot find matching parameter at DW_TAG_call_site 0x10000690 at main
$1 = <optimized out>
...
due to the fact that the call site has no call site parameters.

The call site does have a reference to the corresponding function f1, with
parameter i, for which we find location list entries:
...
  0037 1000067c 10000680 (DW_OP_reg3 (r3))
  004a 10000680 10000690 (DW_OP_GNU_entry_value: (DW_OP_reg3 (r3));
                          DW_OP_stack_value)
...
and we could use the fact that the current pc is in the 1000067c-10000680
range, and that that the range starts at the start of the function, to deduce
that DW_OP_GNU_entry_value: (DW_OP_reg3 (r3)) == DW_OP_reg3 (r3).
But that's a non-trivial enhancement, filed as enhancement PR symtab/29836.

Fix this by allowing <optimized out> for target powerpc and the gcc compiler.

Reviewed-By: Carl Love <cel@us.ibm.com>
Tested-By: Carl Love <cel@us.ibm.com>
PR testsuite/29813
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29813