Disable relaxation in the presence of .cfi directives

The GAS internals perform intra-section label subtractions, which
conflict with our linker relaxations.  So don't relax when emitting
them.

GCC can emit DWARF directly, so this doesn't actually prevent
relaxation in the common case.

gas/config/tc-riscv.c

@@ -2408,6 +2408,10 @@ RISC-V options:\n\
 void
 riscv_cfi_frame_initial_instructions (void)
 {
+  /* The generic DWARF code assumes that intra-section labels can be
+     subtracted at assembly time, which precludes relaxation.  */
+  riscv_opts.relax = 0;
+
   cfi_add_CFA_def_cfa_register (X_SP);
 }