Remove incorrect use of static variable

It prevents modeling mixed RV32/RV64 systems.

riscv/csrs.cc

@@ -455,12 +455,11 @@ reg_t base_status_csr_t::compute_sstatus_write_mask() const noexcept {
 }
 
 reg_t base_status_csr_t::adjust_sd(const reg_t val) const noexcept {
-  // This uses get_const_xlen() instead of get_xlen() not only because
-  // the variable is static, so it's only called once, but also
+  // This uses get_const_xlen() instead of get_xlen()
   // because the SD bit moves when XLEN changes, which means we would
   // need to call adjust_sd() on every read, instead of on every
   // write.
-  static const reg_t sd_bit = proc->get_const_xlen() == 64 ? SSTATUS64_SD : SSTATUS32_SD;
+  const reg_t sd_bit = proc->get_const_xlen() == 64 ? SSTATUS64_SD : SSTATUS32_SD;
   if (((val & SSTATUS_FS) == SSTATUS_FS) ||
       ((val & SSTATUS_VS) == SSTATUS_VS) ||
       ((val & SSTATUS_XS) == SSTATUS_XS)) {