Avoid undefined behaviour with signed expressions

Avoid undefined behaviour with signed expressions

	PR 17453
bfd/
	* libbfd.c (COERCE16, COERCE32, COERCE64): Use unsigned types.
	(EIGHT_GAZILLION): Delete.
binutils/
	* dwarf.c (read_leb128): Avoid signed overflow.
	(read_debug_line_header): Likewise.
gas/
	* config/tc-i386.c (fits_in_signed_long): Use unsigned param and
	expression to avoid signed overflow.
	(fits_in_signed_byte, fits_in_unsigned_byte, fits_in_unsigned_word,
	fits_in_signed_word, fits_in_unsigned_long): Similarly.
	* expr.c (operand <'-'>): Avoid signed overflow.
	* read.c (s_comm_internal): Likewise.