Hi all,

second patch of the series adding 'Tag_BTI_extension' build attribute
support.

Here follows its semantic:

  0 The user did not permit this entity to use BTI instructions
  1 The user permitted this entity to use BTI instructions in the NOP space
  2 The user permitted this entity to use BTI instructions in the NOP and in the non-NOP space

See:
- Armv8.1-M Pointer Authentication and Branch Target Identification Extension [1]
- Armv8-M Architecture Reference Manual [2]

Regtested clean on current master.

Best Regards

  Andrea

[1] <https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension>
[2] <https://developer.arm.com/documentation/ddi0553/latest>