Relax BTI PLT requirement in aaelf64

The ABI states that all PLT entries must start with BTI when
the DT_AARCH64_BTI_PLT dynamic tag is present. This is too
strict as the static linker can prove that some PLT entries
are never indirectly called so do not need to start with a
BTI. Both LLD and GNU ld do this.

Relax the wording to bring the specification in line with
implementations.

#197

Author: smithp35

aaelf64/aaelf64.rst

@@ -1881,11 +1881,12 @@ Dynamic Linking
 Custom PLTs
 ^^^^^^^^^^^^
 
-- To support Branch Target Identification mechanism, in the presence of a
-  ``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` all PLT entries generated
-  by the linker must have a BTI instruction as the first instruction. The
-  linker must add the ``DT_AARCH64_BTI_PLT`` (`AArch64 specific dynamic array tags`_) tag to
-  the dynamic section.
+- To support Branch Target Identification mechanism, in the presence
+  of a ``GNU_PROPERTY_AARCH64_FEATURE_1_BTI`` all PLT entries
+  generated by the linker that can be called indirectly must have a
+  BTI instruction as the first instruction. The linker must add the
+  ``DT_AARCH64_BTI_PLT`` (`AArch64 specific dynamic array tags`_) tag
+  to the dynamic section.
 
 - To support Pointer Authentication, PLT entries generated
   by the linker can have an authenticating instruction as the final