boot: init lregs and fregs

am/src/nemu/isa/riscv/boot/start.S

@@ -4,7 +4,35 @@
 
 #define MSTATUS_FS 0x00006000
 
+.macro set_reg_zero reg_idx
+  mv x\reg_idx, zero
+.endm
+
+.macro set_freg_zero freg_idx
+  fmv.w.x f\freg_idx, zero
+.endm
+
+.macro init_regs
+  .altmacro
+  .set i, 1
+  .rept 31
+    set_reg_zero %i
+    .set i, i+1
+  .endr
+.endm
+
+.macro init_fregs
+  .set i, 0
+  .rept 32
+    set_freg_zero %i
+    .set i, i+1
+  .endr
+.endm
+
 _start:
+  init_regs
+  init_fregs
+
   mv s0, zero
   la sp, _stack_pointer
   li a0, MSTATUS_FS & (MSTATUS_FS >> 1)