amd64/sysv: unbreak env calls

Env calls were disfunctional from the
start. This fixes them on amd64, but
they remain to do on arm64. A new
test shows how to use them.

test/env.ssa

@@ -0,0 +1,21 @@
+# sanity checks for env calls
+
+function l $epar(env %e, l %i) {
+@start
+	%x =l add %e, %i
+	ret %x
+}
+
+export function l $earg(l %a, l %b) {
+@start
+	%r1 =l call $epar(env %a, l %b)
+	# okay to call a regular function
+	# with an env argument
+	%r2 =l call $labs(env 113, l %r1)
+	ret %r2
+}
+
+# >>> driver
+# extern long earg(long, long);
+# int main(void) { return !(earg(2, -44) == 42); }
+# <<<