aarch64: Fix missing BTI instruction in trampolines

aarch64: Fix missing BTI instruction in trampolines

If two functions require trampolines, and the first has BTI enabled
while the second doesn't, the generated template will be lacking
a BTI instruction.  This patch fixes this by always adding a BTI
instruction, which is safe as BTI instructions are ignored on
unsupported architecture versions.

2020-07-01  Omar Tahir  <omar.tahir@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_asm_trampoline_template): Always
	generate a BTI instruction.

gcc/testsuite/
	* gcc.target/aarch64/bti-4.c: New test.