libgcc/CET: Add _CET_ENDBR to __stack_split_initialize

libgcc/CET: Add _CET_ENDBR to __stack_split_initialize

Program received signal SIGSEGV, Segmentation fault.
__stack_split_initialize ()
    at /export/gnu/import/git/sources/gcc/libgcc/config/i386/morestack.S:751
751		leaq	-16000(%rsp),%rax	# We should have at least 16K.
Missing separate debuginfos, use: dnf debuginfo-install libgcc-8.0.1-0.21.0.fc28.x86_64
(gdb) disass
Dump of assembler code for function __stack_split_initialize:
=> 0x0000000000402858 <+0>:	lea    -0x3e80(%rsp),%rax
   0x0000000000402860 <+8>:	mov    %rax,%fs:0x70
   0x0000000000402869 <+17>:	sub    $0x8,%rsp
   0x000000000040286d <+21>:	mov    %rsp,%rdi
   0x0000000000402870 <+24>:	mov    $0x3e80,%esi
   0x0000000000402875 <+29>:	callq  0x401810 <__generic_morestack_set_initial_sp>
   0x000000000040287a <+34>:	add    $0x8,%rsp
   0x000000000040287e <+38>:	retq
End of assembler dump.
(gdb)

This patch adds the missing ENDBR to __stack_split_initialize.

	PR libgcc/85379
	* config/i386/morestack.S (__stack_split_initialize): Add
	_CET_ENDBR.

From-SVN: r259497