Reuse sstatus bitmask for mstatus bits

To reduce repetition in code.

This has one functional effect that I can see: before this change, on
a config that supports floating point but not S-mode or Vector,
writing misa.F=0 would freeze the FS bits in mstatus. I assume this
was an accident and never the intended behavior, though I can't find
anything in the privileged spec about this.

Now the FS bits will always be writable regardless of misa.F, if the
bits exist at all. (Because we now call extension_enabled('F') only
once, at reset, instead of every mstatus write.)

riscv/csrs.cc

@@ -412,19 +412,13 @@ bool mstatus_csr_t::unlogged_write(const reg_t val) noexcept {
       ))
     proc->get_mmu()->flush_tlb();
 
-  bool has_fs = proc->extension_enabled('S') || proc->extension_enabled('F')
-              || proc->extension_enabled_const('V');
-  bool has_vs = proc->extension_enabled_const('V');
   bool has_mpv = proc->extension_enabled('S') && proc->extension_enabled('H');
   bool has_gva = has_mpv;
 
-  reg_t mask = MSTATUS_MIE | MSTATUS_MPIE | MSTATUS_MPRV | MSTATUS_MPP
-             | (proc->extension_enabled('S') ? (MSTATUS_SIE | MSTATUS_SPIE | MSTATUS_SPP) : 0)
-             | MSTATUS_TW | MSTATUS_TSR
-             | (has_page ? (MSTATUS_MXR | MSTATUS_SUM | MSTATUS_TVM) : 0)
-             | (has_fs ? MSTATUS_FS : 0)
-             | (has_vs ? MSTATUS_VS : 0)
-             | (proc->any_custom_extensions() ? MSTATUS_XS : 0)
+  reg_t mask = sstatus_write_mask
+             | MSTATUS_MIE | MSTATUS_MPIE | MSTATUS_MPRV
+             | MSTATUS_MPP | MSTATUS_TW | MSTATUS_TSR
+             | (has_page ? MSTATUS_TVM : 0)
              | (has_gva ? MSTATUS_GVA : 0)
              | (has_mpv ? MSTATUS_MPV : 0);
 

riscv/csrs.h

@@ -220,7 +220,7 @@ typedef std::shared_ptr<vsstatus_csr_t> vsstatus_csr_t_p;
 //    state.sstatus. When complete, all references to sstatus that
 //    need to be virtualized will be through this object.
 // 3. [done] Convert mstatus into a csr_t subclass.
-// 4. Refactor common code into base class.
+// 4. [done] Refactor common code into base class.
 // 5. [done] Convert sstatus to a virtualized_csr_t, with a
 //    nonvirtual_sstatus of type sstatus_proxy_csr_t, and
 //    simultaneously remove the swapping of mstatus & vsstatus from