aarch64: s390: x86_64: sframe: adjust flex fde hook implementation

aarch64: s390: x86_64: sframe: adjust flex fde hook implementation

A previous commit added new backend hook sframe_support_flex_fde_p by
defining a new function for each backend that supports SFrame stack
trace format:

  commit 2f9b3987db53d7e0606f89bfe2527dd3d0915568
  [SFrame-V3] gas: sframe: add new backend hook
  sframe_support_flex_fde_p for FLEX FDEs

As pointed out in a review comment, simply providing the definition to
use true/false (as applicable) is sufficient for the purpose, and helps
generate better code.  So do that.
https://inbox.sourceware.org/binutils/80404871-53b4-4f5c-9d86-7a3a4d0a920e@suse.com/

ChangeLog:
	* gas/config/tc-aarch64.c (aarch64_support_flex_fde_p): Remove.
	* gas/config/tc-aarch64.h (aarch64_support_flex_fde_p): Remove.
	(sframe_support_flex_fde_p): Define to false.
	* gas/config/tc-i386.c (x86_support_flex_fde_p): Remove.
	* gas/config/tc-i386.h (x86_support_flex_fde_p): Remove.
	(sframe_support_flex_fde_p): Define to true.
	* gas/config/tc-s390.c (s390_support_flex_fde_p): Remove.
	* gas/config/tc-s390.h (s390_support_flex_fde_p): Remove.
	(sframe_support_flex_fde_p): Define to true.