With this patch we get rid of the usage of the glibc-internal symbol
__tls_get_addr_internal.

If build with multilib, the file
gcc/libphobos/libdruntime/config/systemz/get_tls_offset.S is used
for both configurations: systemz and s390.
Therefore both implementations are now in the systemz file which
uses an "#ifdef __s390x__" in order to distinguish both cases.
The s390 file is just including the systemz one.

libphobos/ChangeLog:

2020-04-07  Robin Dapp  <rdapp@linux.ibm.com>
            Stefan Liebler  <stli@linux.ibm.com>

	* configure: Regenerate.
	* libdruntime/Makefile.am: Add s390x and s390.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/config/s390/get_tls_offset.S: New file.
	* libdruntime/config/systemz/get_tls_offset.S: New file.
	* libdruntime/gcc/sections/elf_shared.d: Use ibmz_get_tls_offset.
	* m4/druntime/cpu.m4: Add s390x and s390.