[SFrame-V3] gas: sframe: add new backend hook sframe_support_flex_fde_p for FLEX FDEs

For the newly added SFrame FDE type SFRAME_FDE_TYPE_FLEX, add a new
backend hook so that the respective ABI/arch can opt out of the
generation of SFRAME_FDE_TYPE_FLEX in GAS.

AArch64 is an example of an ABI for which SFrame is supported, but one
that does not need the flexible FDE representation for any of the
current usecases currently.

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

gas/
	* config/tc-aarch64.c (aarch64_support_flex_fde_p): New
	definition.
	* config/tc-aarch64.h (aarch64_support_flex_fde_p): New
	declaration.
	(sframe_support_flex_fde_p): Define.
	* config/tc-i386.c (x86_support_flex_fde_p): New definition.
	* config/tc-i386.h (x86_support_flex_fde_p): New declaration.
	(sframe_support_flex_fde_p): Define.
	* config/tc-s390.c (s390_support_flex_fde_p): New definition.
	* config/tc-s390.h (s390_support_flex_fde_p): New declaration.
	(sframe_support_flex_fde_p): Define.