We no longer use the so called front-end SJLJ exception mechanism, so
get rid of it.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* ali.adb, ali.ads, bcheck.adb, exp_ch11.adb, fe.h,
	gnat1drv.adb, opt.adb, opt.ads, targparm.adb, targparm.ads,
	lib-writ.adb: Get rid of Frontend_Exceptions processing.
	* libgnat/system-aix.ads, libgnat/system-darwin-arm.ads,
	libgnat/system-darwin-ppc.ads, libgnat/system-darwin-x86.ads,
	libgnat/system-djgpp.ads, libgnat/system-dragonfly-x86_64.ads,
	libgnat/system-freebsd.ads, libgnat/system-hpux-ia64.ads,
	libgnat/system-hpux.ads, libgnat/system-linux-alpha.ads,
	libgnat/system-linux-arm.ads, libgnat/system-linux-hppa.ads,
	libgnat/system-linux-ia64.ads, libgnat/system-linux-m68k.ads,
	libgnat/system-linux-mips.ads, libgnat/system-linux-ppc.ads,
	libgnat/system-linux-riscv.ads, libgnat/system-linux-s390.ads,
	libgnat/system-linux-sh4.ads, libgnat/system-linux-sparc.ads,
	libgnat/system-linux-x86.ads, libgnat/system-lynxos178-ppc.ads,
	libgnat/system-lynxos178-x86.ads, libgnat/system-mingw.ads,
	libgnat/system-qnx-aarch64.ads, libgnat/system-rtems.ads,
	libgnat/system-solaris-sparc.ads,
	libgnat/system-solaris-x86.ads,
	libgnat/system-vxworks-arm-rtp-smp.ads,
	libgnat/system-vxworks-arm-rtp.ads,
	libgnat/system-vxworks-arm.ads,
	libgnat/system-vxworks-e500-kernel.ads,
	libgnat/system-vxworks-e500-rtp-smp.ads,
	libgnat/system-vxworks-e500-rtp.ads,
	libgnat/system-vxworks-ppc-kernel.ads,
	libgnat/system-vxworks-ppc-rtp-smp.ads,
	libgnat/system-vxworks-ppc-rtp.ads,
	libgnat/system-vxworks-x86-kernel.ads,
	libgnat/system-vxworks-x86-rtp-smp.ads,
	libgnat/system-vxworks-x86-rtp.ads,
	libgnat/system-vxworks7-aarch64-rtp-smp.ads,
	libgnat/system-vxworks7-aarch64.ads,
	libgnat/system-vxworks7-arm-rtp-smp.ads,
	libgnat/system-vxworks7-arm.ads,
	libgnat/system-vxworks7-e500-kernel.ads,
	libgnat/system-vxworks7-e500-rtp-smp.ads,
	libgnat/system-vxworks7-e500-rtp.ads,
	libgnat/system-vxworks7-ppc-kernel.ads,
	libgnat/system-vxworks7-ppc-rtp-smp.ads,
	libgnat/system-vxworks7-ppc-rtp.ads,
	libgnat/system-vxworks7-ppc64-kernel.ads,
	libgnat/system-vxworks7-ppc64-rtp-smp.ads,
	libgnat/system-vxworks7-x86-kernel.ads,
	libgnat/system-vxworks7-x86-rtp-smp.ads,
	libgnat/system-vxworks7-x86-rtp.ads,
	libgnat/system-vxworks7-x86_64-kernel.ads,
	libgnat/system-vxworks7-x86_64-rtp-smp.ads: Remove
	Frontend_Exceptions line.
	* gcc-interface/decl.cc, gcc-interface/trans.cc
	(gnat_to_gnu_entity, gnat_to_gnu_subprog_type, gigi,
	gnat_to_gnu): Remove Front_End_SJLJ processing and always assume
	Back_End_Exceptions.