arm64: asm: Override SYM_FUNC_START when building the kernel with BTI

arm64: asm: Override SYM_FUNC_START when building the kernel with BTI

When the kernel is built for BTI override SYM_FUNC_START and related macros
to add a BTI landing pad to the start of all global functions, ensuring that
they are BTI safe. The ; at the end of the BTI_x macros is for the
benefit of the macro-generated functions in xen-hypercall.S.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200506195138.22086-4-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>