gas: ld: sframe: add new internal header

gas: ld: sframe: add new internal header

for SFRAME_V2_GNU_AS_LD_ENCODING_FLAGS.

The intention of creating an abstraction like
SFRAME_V2_GNU_AS_LD_ENCODING_FLAGS is to address the concern that there
should be a central place to enforce harmonious flags between GNU as and
ld.  At the moment, the only flag that needs to be enforced is
SFRAME_F_FDE_FUNC_START_PCREL.

sframe.h and sframe-api.h are installed headers by libsframe for the
specification and implementation respectively.  Adding a definition like
SFRAME_V2_GNU_AS_LD_ENCODING_FLAGS does not fit in either.  Create a
new internal header instead to keep the definition uncoupled from
sframe.h and sframe-api.h.  Rename the previously added
SFRAME_F_LD_MUSTHAVE_FLAGS to define the new
SFRAME_V2_GNU_AS_LD_ENCODING_FLAGS.

bfd/
	* elf-sframe.c (_bfd_elf_merge_section_sframe): Use the new
	internal header and SFRAME_V2_GNU_AS_LD_ENCODING_FLAGS.
gas/
	* gen-sframe.c (output_sframe_internal): Likewise.
include/
	* sframe-api.h (SFRAME_F_LD_MUSTHAVE_FLAGS): Move from..
	* sframe-internal.h: ..to here.  New file.