include: libsframe: Add API to get RA undefined

include: libsframe: Add API to get RA undefined

SFrame FREs without any offsets will later be used to represent an
undefined return address (RA) in SFrame.  This API can then be used,
for instance by libsframe when dumping SFrame stack trace information
(e.g. in objdump and readelf), to test for RA undefined.  Other users
of libsframe need the same capability.

include/
	* sframe-api.h (sframe_fre_get_ra_undefined_p): New declaration.

libsframe/
	* libsframe.ver (sframe_fre_get_ra_undefined_p): List new API.
	* sframe.c (sframe_fre_get_ra_undefined_p): New definition.

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