arm: Fix missing bti instruction for virtual thunks

Adds missing bti instruction at the beginning of a virtual
thunk, when bti is enabled.

gcc/ChangeLog:

	* config/arm/arm.cc (arm_output_mi_thunk): Emit
	insn for bti_c when bti is enabled.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Add v8_1_m_main_pacbti.
	* g++.target/arm/bti_thunk.C: New test.