bfd: aarch64: Avoid BTI stub for a PLT that has BTI

bfd: aarch64: Avoid BTI stub for a PLT that has BTI

We decide to emit BTI stubs based on the instruction at the target
location. But PLT code is generated later than the stubs so we always
read 0 which is not a valid BTI.

Fix the logic to special case the PLT section: this is code the linker
generates so we know when it will have BTI.

This avoids BTI stubs in large executables where the PLTs have them
already. An alternative is to never emit BTI stubs for PLTs, instead
use BTI in the PLT if a library gets too big, however that may be
more tricky given the ordering of PLT sizing and stub insertion.

Related to bug 30957.