Introduce BTI support in ROMLIB

When TF-A is compiled with BTI enabled, the branches in the ROMLIB
jumptable must be preceded by a "bti j" instruction.

Moreover, when the additional "bti" instruction is inserted, the
jumptable entries have a distance of 8 bytes between them instead of 4.
Hence, the wrappers are also modified accordinly.

If TF-A is compiled without BTI enabled, the ROMLIB jumptable and
wrappers are generated as before.

Change-Id: Iaa59897668f8e59888d39046233300c2241d8de7
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>