[BFD, AArch64] Improve bti/pac plts.

[BFD, AArch64] Improve bti/pac plts.

This patch aims to improve the definitions of BTI and PAC based PLTs.
The following changes are made:
   * PLT0 does not need PAC instructions since the PLTGOT[2] (and PLTGOT[1])
     are readonly so they cannot be corrupted at runtime. Thus both PAC plt0
     and BTI+PAC plt0 are removed and we can use basic plt0 and BTI plt0
     instead, respectively.
   * We can remove the extra padding nops when we add the new bti instructions.
     BTI plt0 and BTI TLSDESC plt are updated.
   * For better performance PLTn could be padded to 24bytes. Both BTI pltn and
     PAC pltn are updated.

*** bfd/ChangeLog ***

2019-04-25  Sudakshina Das  <sudi.das@arm.com>

	* elfnn-aarch64.c (PLT_BTI_ENTRY_SIZE): Remove.
	(PLT_BTI_TLSDESC_ENTRY_SIZE): Remove.
	(PLT_PAC_ENTRY_SIZE, PLT_BTI_PAC_ENTRY_SIZE): Remove.
	(PLT_BTI_SMALL_ENTRY_SIZE, PLT_PAC_SMALL_ENTRY_SIZE): Update.
	(elfNN_aarch64_small_plt0_pac_entry): Remove.
	(elfNN_aarch64_small_plt0_bti_pac_entry): Remove.
	(elfNN_aarch64_small_plt0_bti_entry): Update.
	(elfNN_aarch64_small_plt_bti_entry): Update.
	(elfNN_aarch64_small_plt_pac_entry): Update.
	(elfNN_aarch64_tlsdesc_small_plt_bti_entry): Update.
	(setup_plt_values): Setup new entries.
	(elfNN_aarch64_finish_dynamic_sections): Remove size change.
	(elfNN_aarch64_plt_sym_val): Likewise.

*** ld/ChangeLog ***

2019-04-25  Sudakshina Das  <sudi.das@arm.com>

	* testsuite/ld-aarch64/bti-pac-plt-1.d: Update.
	* testsuite/ld-aarch64/bti-pac-plt-2.d: Update.
	* testsuite/ld-aarch64/bti-plt-1.d: Update.
	* testsuite/ld-aarch64/bti-plt-3.d: Update.
	* testsuite/ld-aarch64/bti-plt-5.d: Update.
	* testsuite/ld-aarch64/pac-plt-1.d: Update.
	* testsuite/ld-aarch64/pac-plt-2.d: Update.