dwarf: Read register number as unsigned in DW_CFA_def_cfa*

dwarf: Read register number as unsigned in DW_CFA_def_cfa*

When displaying the .debug_frame section, the register numbers in the
DW_CFA_def_cfa* statements are read as signed numbers.  I have come
across a target that has register 121, encoded as 0x79 in unsigned LEB128.
Interpreting this as signed results in -7, which makes readelf display
"r-7".

The DWARF5 standard (6.4.2.2) states that the register numbers should be
treated as unsigned LEB128.

Simply replacing READ_SLEB with READ_ULEB resulted in warnings like
these:

/home/emaisin/src/binutils-gdb/binutils/dwarf.c: In function ‘display_debug_frames’:
/home/emaisin/src/binutils-gdb/binutils/dwarf.c:355:17: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
       if ((var) != _val)     \
                 ^
/home/emaisin/src/binutils-gdb/binutils/dwarf.c:7866:8: note: in expansion of macro ‘READ_ULEB’
        READ_ULEB (fc->cfa_reg);
        ^
... so I also changed Frame_Chunk::cfa_reg to an unsigned int.

binutils/ChangeLog:

	* dwarf.c (struct Frame_Chunk) <cfa_reg>: Change type to
	unsigned int.
	(display_debug_frames): Read CFA reg as an unsigned number.