Update elf_i386_compute_jump_table_size

Commit e1f98742 changed how
next_tls_desc_index was set up.  This patch updates
elf_i386_compute_jump_table_size to use elf.srelplt->reloc_count
instead of next_tls_desc_index.

bfd/

	PR ld/17057
	* elf32-i386.c (elf_i386_compute_jump_table_size): Replace
	next_tls_desc_index with elf.srelplt->reloc_count.

ld/testsuite/

	PR ld/17057
	* ld-i386/i386.exp: Run pr17057.
	* ld-i386/pr17057.d: New file.
	* ld-i386/pr17057.s: Likewise.