static dtb: add BTI note

[br]

Summary

Add BTI note to fdt_static_dtb.o

• resolves "Branch Target exception" panic on FDT systems with static DTB
• also required by bti_report linker feature

Test Plan

tested on cix cd8160

readelf --notes ../arm64.aarch64/sys/GENERIC/fdt_static_dtb.o
Displaying notes found in: .note.gnu.property
  Owner                Data size 	Description
  GNU                  0x00000010	NT_GNU_PROPERTY_TYPE_0 (property note)
    Properties:    aarch64 feature: BTI, PAC

[andrew]

How does it fix the Branch Target exception? The kernel doesn't care about the note, it's just there to ensure all source files are built with BTI enabled.

[br]

In D55636#1273367, @andrew wrote:

How does it fix the Branch Target exception? The kernel doesn't care about the note, it's just there to ensure all source files are built with BTI enabled.

I will check, but could something happen with landing pads in the beginning of functions if one of the objects used for linkage has no BTI note?

[andrew]

The change is correct. I think it is more likely that rebuilding the kernel with a consistent toolchain is the fix for the BTI exception.

[br]

Same toolchain (latest), but without this change a few functions don't have landing pad: copyin/copyout and sched_idletd.

[andrew]

I see, the issue is the generated ifunc trampolines only have BTI landing pads when the note is in all object files.