include: libsframe: add APIs for offsetof FDE func start addr field

include: libsframe: add APIs for offsetof FDE func start addr field

These APIs will be later used by the linker to arrange SFrame FDEs in
the output SFrame section.

include/
        * sframe-api.h (sframe_decoder_get_offsetof_fde_start_addr): New
	declaration.
        (sframe_encoder_get_offsetof_fde_start_addr): Likewise.

libsframe/
        * libsframe.ver: List the new APIs.
        * sframe.c (sframe_decoder_get_offsetof_fde_start_addr): New
	definition.
        (sframe_encoder_get_offsetof_fde_start_addr): Likewise.

---
[No changes in V4]

[No changes in V3]

[Changes in V2]
  - Keep the function level comments in sync.
  - Use an additional offsetof to convey the semantics clearly.
[End of changes in V2]