IBM Z: Fix PR96308

For the testcase a symbol with a TLS reloc and an unary minus is being
generated.  The backend didn't handle this correctly.

In s390_cannot_force_const_mem an unary minus on a symbolic constant
is rejected now since gas would not allow this.

legitimize_tls_address now makes the NEG rtx the outermost operation
by pulling it out of the CONST rtx.

gcc/ChangeLog:

	PR target/96308
	* config/s390/s390.c (s390_cannot_force_const_mem): Reject an
	unary minus for everything not being a numeric constant.
	(legitimize_tls_address): Move a NEG out of the CONST rtx.

gcc/testsuite/ChangeLog:

	PR target/96308
	* g++.dg/pr96308.C: New test.