aarch64: set sh_entsize of .plt to 0

On aarch64 the first PLT entry is 32 bytes, subsequent entries
are 16 bytes by default but can be 24 bytes with BTI or with
PAC-PLT.

sh_entsize of .plt was set to the PLT entry size, so in some
cases sh_size % sh_entsize != 0, which breaks some tools.

Note that PLT0 (and the TLSDESC stub code which is also in the
PLT) were historically not padded up to meet the sh_size
requirement, but to ensure that PLT stub code is aligned on
cache lines. Similar layout is present on other targets too
which just happens to make sh_size a multiple of sh_entsize and
it is not expected that sh_entsize of .plt is used for anything.

This patch sets sh_entsize of .plt to 0: the section does not
hold a table of fixed-size entries so other values are not
conforming in principle to the ELF spec.

bfd/ChangeLog:

	Backport from mainline.
	2020-07-30  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR ld/26312
	* elfnn-aarch64.c (elfNN_aarch64_init_small_plt0_entry): Set sh_entsize
	to 0.
	(elfNN_aarch64_finish_dynamic_sections): Remove sh_entsize setting.

(cherry picked from commit 4d3bb356)