IBM Z: Spectre: Prevent thunk cfi to be emitted with -fno-dwarf2-cfi-asm

IBM Z: Spectre: Prevent thunk cfi to be emitted with -fno-dwarf2-cfi-asm

The CFI magic we emit as part of the indirect branch thunks in order to
have somewhat sane unwind information must not be emitted with
-fno-dwarf2-cfi-asm.

gcc/ChangeLog:

2018-04-12  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/s390/s390.c (s390_output_indirect_thunk_function): Check
	also for flag_dwarf2_cfi_asm.

gcc/testsuite/ChangeLog:

2018-04-12  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* gcc.target/s390/nobp-no-dwarf2-cfi.c: New test.

From-SVN: r259340