libsframe: relax the assertion limit for fre_start_addr

Fix PR ld/33131 Failed assertion when linking gccgo

Make amendments in both sframe_decoder_get_fre and
sframe_encoder_add_fre.

Since GNU as and the dw2gencfi code generally accepts such CFI, its best
to allow in SFrame FREs too.

libsframe/
	PR ld/33131.
	* sframe.c (sframe_decoder_get_fre): Relax the assertion a bit.
	(sframe_encoder_add_fre): Likewise.