env: use writev to add the newline

this allows us to print each line in a single, atomic write call,
without modifying or buffering it.

env.s

@@ -27,10 +27,18 @@ _start:
     sll t0, s0, 3 # argc*8
     add s1, s1, t0
 
+    # allocate space for an iovec
+    addi sp, sp, -(8+8)*2
+
+    la t0, newline
+    sd t0, 16(sp)
+    li t1, 1
+    sd t1, 24(sp)
 loop:
     # check for null pointer, which signals the end of envp
     ld a1, 0(s1)
     beqz a1, end
+    sd a1, 0(sp)
 
     # compute length of string
     mv t1, a1
@@ -39,7 +47,9 @@ loop:
     addi t1, t1, 1
     bnez t0, strlen
     sub a2, t1, a1
+    sd a2, 8(sp)
 
+    /*
     # print string
     li a7, 64
     li a0, 1
@@ -53,6 +63,13 @@ loop:
     la a1, newline
     li a2, 1
     ecall
+    */
+    li a7, 66 # sys_writev
+    li a0, 1
+    mv a1, sp
+    li a2, 2
+    ecall
+    bltz a0, write_error
 
     # increment envp and loop
     addi s1, s1, 8