sframe: gas: libsframe: define constants and remove magic numbers

Define constants in sframe.h for the various limits associated with the
range of offsets that can be encoded in the start address of an SFrame
FRE. E.g., sframe_frame_row_entry_addr1 is used when start address
offset can be encoded as 1-byte unsigned value.

Update the code in gas to use these defined constants as it checks for
these limits, and remove the usage of magic numbers.

ChangeLog:

	* gas/sframe-opt.c (sframe_estimate_size_before_relax):
	(sframe_convert_frag): Do not use magic numbers.
	* libsframe/sframe.c (sframe_calc_fre_type): Likewise.

include/ChangeLog:

	* sframe.h (SFRAME_FRE_TYPE_ADDR1_LIMIT): New constant.
	(SFRAME_FRE_TYPE_ADDR2_LIMIT): Likewise.
	(SFRAME_FRE_TYPE_ADDR4_LIMIT): Likewise.