gh-139808: Add branch protections for aarch64 in asm_trampoline.S (#130864)

Apply protection against ROP/JOP attacks for aarch64 on asm_trampoline.S.

The BTI flag must be applied in assembler sources for this class
of attacks to be mitigated on newer aarch64 processors.

See also:
https://sourceware.org/annobin/annobin.html/Test-branch-protection.html
and
https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/enabling-pac-and-bti-on-aarch64

Co-authored-by: Victor Stinner <vstinner@python.org>