sjlj.S did not have the GNU property note markup and the BTI c
instructions that are necessary when it is built with branch
protection.

The notes are only added when libitm is built with branch
protection, because old linkers mishandle the note (merge
them incorrectly or emit warnings), the BTI instructions
are added unconditionally.

2020-07-09  Szabolcs Nagy  <szabolcs.nagy@arm.com>

libitm/ChangeLog:

	* config/aarch64/sjlj.S: Add BTI marking and related definitions,
	and add BTI c to function entries.

(cherry picked from commit 319078dad62eba942d33c8975bdcbb09d1c68ba6)