Fix PLT first entry GOT operand calculation.

Fix PLT first entry GOT operand calculation.

Embedding the .plt section in another revealed a bug in the way the
larl operand of the first magic plt entry is being calculated.  Fixed
with the attached patch.

bfd/ChangeLog:

	* elf64-s390.c (elf_s390_finish_dynamic_sections): Subtract plt
	section offset when calculation the larl operand in the first PLT
	entry.

ld/ChangeLog:

	* testsuite/ld-s390/pltoffset-1.dd: New test.
	* testsuite/ld-s390/pltoffset-1.ld: New test.
	* testsuite/ld-s390/pltoffset-1.s: New test.
	* testsuite/ld-s390/s390.exp: Run new test.