static dtb: add BTI note

Add BTI note to fdt_static_dtb.o

• resolves "Branch Target Exception" panic on FDT systems with static DTB due to missing landing pads (bti note has to be in all object files used for linkage)
• also required by bti_report linker feature

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D55636