read_leb128 overflow checking

read_leb128 overflow checking

There is a tiny error left in dwarf.c:read_leb128 after Nick fixed the
signed overflow problem in code I wrote.  It's to do with sleb128
values that have unnecessary excess bytes.  For example, -1 is
represented as 0x7f, the most efficient encoding, but also as
0xff,0x7f or 0xff,0xff,0x7f and so on.  None of these sequences
overflow any size signed value, but read_leb128 will report an
overflow given enough excess bytes.  This patch fixes that problem,
and since the proper test for signed values with excess bytes can
easily be adapted to also test a sleb byte with just some bits that
overflow the result, I changed the code to not use signed right
shifts.  (The C standard ISO/IEC 9899:1999 6.5.7 says signed right
shifts of negative values have an implementation defined value.  A
long time ago I even used a C compiler for a certain microprocessor
that always did unsigned right shifts.  Mind you, it is very unlikely
to be compiling binutils with such a compiler.)

bfd/
	* wasm-module.c: Guard include of limits.h.
	(CHAR_BIT): Provide backup define.
	(wasm_read_leb128): Use CHAR_BIT to size "result" in bits.
	Correct signed overflow checking.
opcodes/
	* wasm32-dis.c: Include limits.h.
	(CHAR_BIT): Provide backup define.
	(wasm_read_leb128): Use CHAR_BIT to size "result" in bits.
	Correct signed overflow checking.
binutils/
	* dwarf.c: Include limits.h.
	(CHAR_BIT): Provide backup define.
	(read_leb128): Use CHAR_BIT to size "result" in bits.  Correct
	signed overflow checking.
	* testsuite/binutils-all/pr26548.s,
	* testsuite/binutils-all/pr26548.d,
	* testsuite/binutils-all/pr26548e.d: New tests.
	* testsuite/binutils-all/readelf.exp: Run them.
	(readelf_test): Drop unused "xfails" parameter.  Update all uses.