sframe: fix the defined SFRAME_FRE_TYPE_*_LIMIT constants

An earlier commit 3f107464 defined the SFRAME_FRE_TYPE_*_LIMIT
constants.  These constants are used (by gas and libsframe) to pick an
SFrame FRE type based on the function size.  Those constants, however,
were buggy, causing the generated SFrame sections to be bloated as
SFRAME_FRE_TYPE_ADDR2/SFRAME_FRE_TYPE_ADDR4 got chosen more often than
necessary.

gas/
	* sframe-opt.c (sframe_estimate_size_before_relax): Use
	typecast.
	(sframe_convert_frag): Likewise.

libsframe/
	* sframe.c (sframe_calc_fre_type): Use a more
	appropriate type for argument.  Adjust the check for
	SFRAME_FRE_TYPE_ADDR4_LIMIT to keep it warning-free but
	meaningful.

include/
	* sframe-api.h (sframe_calc_fre_type): Use a more appropriate
	type for the argument.
	* sframe.h (SFRAME_FRE_TYPE_ADDR1_LIMIT): Correct the constant.
	(SFRAME_FRE_TYPE_ADDR2_LIMIT): Likewise.
	(SFRAME_FRE_TYPE_ADDR4_LIMIT): Likewise.