backends: Make aarch64_regs.c checkable by -Wprintf
libdw: Read DW_AT_decl_file/line/column as unsigned

Section 2.14 of the DWARF v3 & v4 standards specifies that all three
declaration coordinates are unsigned integer constants.  DWARF v2 did
not specify signedness.  Now dwarf_decl_* use dwarf_formudata to read
these values.

Also, an assertion on the range of line/column is now a handled error,
setting DWARF_E_INVALID_DWARF for values greater than INT_MAX.

Signed-off-by: Josh Stone <jistone@redhat.com>