x86-64: Rename .plt.bnd to .plt.sec

x86-64: Rename .plt.bnd to .plt.sec

Rename .plt.bnd to .plt.sec to indicate that this is used as the second
PLT section.  There is no change in run-time behavior.  We also scan the
.plt.sec section to synthesize PLT symbols.

bfd/

	* elf64-x86-64.c (elf_x86_64_link_hash_entry): Rename plt_bnd
	to plt_second.
	(elf_x86_64_link_hash_table): Rename plt_bnd/plt_bnd_eh_frame
	to plt_second/plt_second_eh_frame.
	(elf_x86_64_link_hash_newfunc): Updated.
	(elf_x86_64_allocate_dynrelocs): Likewise.
	(elf_x86_64_size_dynamic_sections): Likewise.
	(elf_x86_64_relocate_section): Likewise.
	(elf_x86_64_finish_dynamic_symbol): Likewise.
	(elf_x86_64_finish_dynamic_sections): Likewise.
	(elf_x86_64_plt_type): Rename plt_bnd to plt_second.
	(elf_x86_64_get_synthetic_symtab): Updated.  Also scan the
	.plt.sec section.
	(elf_backend_setup_gnu_properties): Updated.  Create the
	.plt.sec section instead of the .plt.sec section.

ld/

	* emulparams/elf_x86_64.sh (TINY_READONLY_SECTION): Replace
	.plt.bnd with .plt.sec.
	* testsuite/ld-x86-64/bnd-ifunc-1-now.d: Likewise.
	* testsuite/ld-x86-64/bnd-ifunc-2-now.d: Likewise.
	* testsuite/ld-x86-64/bnd-ifunc-2.d: Likewise.
	* testsuite/ld-x86-64/bnd-plt-1-now.d: Likewise.
	* testsuite/ld-x86-64/bnd-plt-1.d: Likewise.
	* testsuite/ld-x86-64/mpx3.dd: Likewise.
	* testsuite/ld-x86-64/mpx3n.dd: Likewise.
	* testsuite/ld-x86-64/mpx4.dd: Likewise.
	* testsuite/ld-x86-64/mpx4n.dd: Likewise.
	* testsuite/ld-x86-64/plt-main-bnd-now.rd: Likewise.
	* testsuite/ld-x86-64/pr21038b-now.d: Likewise.
	* testsuite/ld-x86-64/pr21038b.d: Likewise.
	* testsuite/ld-x86-64/pr21038c-now.d: Likewise.
	* testsuite/ld-x86-64/pr21038c.d: Likewise.