Color entry block arguments using the function signature.

The arguments to the entry block arrive in registers determined by the
ABI. This information is stored in the signature.

Use a separate function for coloring entry block arguments using the
signature information. We can't handle stack arguments yet.

cranelift/filetests/regalloc/basic.cton

@@ -6,8 +6,7 @@ isa riscv
 function add(i32, i32) {
 ebb0(v1: i32, v2: i32):
     v3 = iadd v1, v2
-; TODO: This shouldn't clobber the link register.
-; check: [R#0c,%x1]
+; check: [R#0c,%x5]
 ; sameln: iadd
     return
 }