arm64: asm: Provide a mechanism for generating ELF note for BTI

arm64: asm: Provide a mechanism for generating ELF note for BTI

ELF files built for BTI should have a program property note section which
identifies them as such. The linker expects to find this note in all
object files it is linking into a BTI annotated output, the compiler will
ensure that this happens for C files but for assembler files we need to do
this in the source so provide a macro which can be used for this purpose.
To support likely future requirements for additional notes we split the
defininition of the flags to set for BTI code from the macro that creates
the note itself.

This is mainly for use in the vDSO which should be a normal ELF shared
library and should therefore include BTI annotations when built for BTI.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200506195138.22086-9-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>