[gdb/contrib] Fix KeyError in dwarf-to-dwarf-assembler.py

Executable outputs/gdb.threads/stepi-over-clone/stepi-over-clone contains the
following DWARF (using readelf -w):
...
    <31c5>   DW_AT_const_value : 8 byte block: ff ff ff ff ff ff ff ff
...
or more explicit using llvm-dwarfdump -v:
...
                DW_AT_const_value [DW_FORM_block1] \
		  (<0x08> ff ff ff ff ff ff ff ff )
...

Before commit 18ad8c1a54d ("Emit DWARF expressions from
dwarf-to-dwarf-assembler.py"), the dwarf-to-dwarf-assembler.py script
generated:
...
DW_AT_const_value { MANUAL: Fill expr list } SPECIAL_expr
...
for this.

But since the commit, we run into a KeyError.

The problem is that we're trying to interpret some random data as a DWARF
expression.

Fix this by special-casing DW_AT_const_value, and generating instead:
...
DW_AT_const_value "\xff\xff\xff\xff\xff\xff\xff\xff" DW_FORM_block1
...
which mimics the style I found in gdb.dwarf2/info-locals-optimized-out.exp:
...
                DW_TAG_variable {
                    DW_AT_name const_bytes
                    DW_AT_type :$int_type_label
                    DW_AT_const_value "\x01\x01\x01\x01" DW_FORM_block1
		}
...

Likewise for DW_AT_discr_list.

Another case for which the KeyError triggers, is DWARF from
outputs/gdb.ada/array_of_variant/p-minimal:
...
 <2><14a4>: Abbrev Number: 20 (DW_TAG_structure_type)
    <14a5>   DW_AT_name        : p__payload_t
    <14a9>   DW_AT_byte_size   : 18 byte block: \
                                 fd 63 14 0 0 97 94 1 99 45 0 0 0 23 7 9 fc 1a \
				 (DW_OP_GNU_variable_value: <0x1463>; \
				  DW_OP_push_object_address; \
				  DW_OP_deref_size: 1; \
				  DW_OP_call4: <0x1421>; \
				  DW_OP_plus_uconst: 7; \
				  DW_OP_const1s: -4; DW_OP_and)
...
because the pyelftools version that I'm using doesn't support
DW_OP_GNU_variable_value yet (supported starting v0.33).

Fix this by catching the KeyError, and falling back to the string case:
...
  DW_AT_byte_size \
  "\xfd\x63\x14\x00\x00\x97\x94\x01\x99\x45\x00\x00\x00\x23\x07\x09\xfc\x1a" \
  DW_FORM_exprloc # Failed to print op as DWARF expression: 0xfd
...