[GCC, AArch64] Fix a couple of bugs in BTI

[GCC, AArch64] Fix a couple of bugs in BTI

While doing more testing I found a couple of issues with my BTI patches.
This patch fixes them:
1) Remove a reference to return address key. The original patch was written
based on a different not yet committed patch ([PATCH 3/3][GCC][AARCH64] Add
support for pointer authentication B key) and I missed out on cleaning this
up. This is hidden behind the configuration option and thus went unnoticed.
2) Add a missed case for adding the BTI instruction in thunk functions.

*** gcc/ChangeLog ***

2019-02-22  Sudakshina Das  <sudi.das@arm.com>

	* config/aarch64/aarch64.c (aarch64_output_mi_thunk): Add bti
	instruction if enabled.
	(aarch64_override_options): Remove reference to return address key.

From-SVN: r269112