gas: sframe: use standard min/max integer constants

Replace the use of custom VALUE_{8|16|32|64}BIT constant definitions
with the standard INT{8|16|32|64}_{MIN|MAX} ones from stdint.h.

Besides improving readability this also fixes the issue that the
smallest representable signed 8/16/32-bit integer value was
erroneously sized as the next larger integer type.  For example
get_offset_size_in_bytes (INT8_MIN) returned 2 instead of 1, due
to INT8_MIN (= -128) != -VALUE_8BIT (= -127):

  (gdb) call get_offset_size_in_bytes (-127)
  $1 = 1
  (gdb) call get_offset_size_in_bytes (-128)
  $2 = 2

gas/
	* gen-sframe.c (VALUE_8BIT, VALUE_16BIT, VALUE_32BIT,
	VALUE_64BIT): Remove.
	(get_offset_size_in_bytes): Use standard min/max integer
	constants.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>