s390: Mention GOTIE20 relocation in TLS related comment

Commit bd1ea41b84b0 introduced the R_390_TLS_GOTIE20 relocation, but
missed to update a comment that mentions the related R_390_TLS_GOTIE12
relocation.

bfd/
	* elf32-s390.c (allocate_dynrelocs): Mention GOTIE20 relocation
	in TLS related comment.
	* elf64-s390.c (allocate_dynrelocs): Likewise.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>