dwarves: Change type of bitfield_offset from uint8_t to int8_t

The dwarves class_member field bitfield_offset represents the dwarf tag
DW_AT_bit_offset. For dwarf2, this field can be negative for little
endian for bitfields in packed data structures which cross type
boundary.

  -bash-4.4$ cat bitfield.c
  struct packed {
    char x1: 1;
    char x2: 3;
    char x3: 3;
    int y1: 7;
    int y2: 20;
  } __attribute__((packed));
  struct packed g;
  -bash-4.4$ gcc -O2 -c -g bitfield.c
  -bash-4.4$ pahole -JV bitfield.o
  File bitfield.o:
  [1] STRUCT packed kind_flag=1 size=5 vlen=5
        x1 type_id=2 bitfield_size=1 bits_offset=0
        x2 type_id=2 bitfield_size=3 bits_offset=1
        x3 type_id=2 bitfield_size=3 bits_offset=4
        y1 type_id=3 bitfield_size=7 bits_offset=7
        y2 type_id=3 bitfield_size=255 bits_offset=16776974
  [2] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
  [3] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
  -bash-4.4$

The above large negative bits_offset and bitfield_size=255 results from
negative bitfield_offset which is interpreted as positive value in btf
encoding.

With this fix, the pahole works properly for BTF:
  -bash-4.4$ pahole -JV bitfield.o
  File bitfield.o:
  [1] STRUCT packed kind_flag=1 size=5 vlen=5
        x1 type_id=2 bitfield_size=1 bits_offset=0
        x2 type_id=2 bitfield_size=3 bits_offset=1
        x3 type_id=2 bitfield_size=3 bits_offset=4
        y1 type_id=3 bitfield_size=7 bits_offset=7
        y2 type_id=3 bitfield_size=20 bits_offset=14
  [2] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
  [3] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
  -bash-4.4$

Note that change bitfield_offset from uint8_t to int8_t is safe as the
maximum int type we support is __int128 and maximum bitfield_offset is
127.

Signed-off-by: Yonghong Song <yhs@fb.com>
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: dwarves@vger.kernel.org
Link: https://www.spinics.net/lists/dwarves/msg00199.html
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

dwarves.h

@@ -831,7 +831,7 @@ struct class_member {
 	uint32_t	 bit_size;
 	uint32_t	 byte_offset;
 	size_t		 byte_size;
-	uint8_t		 bitfield_offset;
+	int8_t		 bitfield_offset;
 	uint8_t		 bitfield_size;
 	uint8_t		 bit_hole;
 	uint8_t		 bitfield_end:1;