[BFD, AArch64] Fix PT_GNU_PROPERTY alignment issue

[BFD, AArch64] Fix PT_GNU_PROPERTY alignment issue

If the new GNU property section was being created by the linker
(this will happen only if none of the inputs have any GNU property
section but the command line to the linker forces a bti with
--force-bti), the alignment of the section and hence the program
header of PT_GNU_PROPERTY type was not being set correctly. This
patch fixes this issue.

bfd/ChangeLog:

2019-06-06  Sudakshina Das  <sudi.das@arm.com>

	* elfxx-aarch64.c (_bfd_aarch64_elf_link_setup_gnu_properties): Set
	alignment of the new gnu property section.

ld/ChangeLog:

2019-06-06  Sudakshina Das  <sudi.das@arm.com>

	* testsuite/ld-aarch64/aarch64-elf.exp: Add new tests.
	* testsuite/ld-aarch64/property-bti-pac4-a.d: New test.
	* testsuite/ld-aarch64/property-bti-pac4-b.d: New test.
	* testsuite/ld-aarch64/property-bti-pac4.s: New test.