Add an enable_e setting for the RV32E instruction set.

This limited RISC-V version only has registers %x0 - %x15.

Make sure the ABI lowering code doesn't use the banned registers for
arguments.

filetests/isa/riscv/abi-e.cton

@@ -0,0 +1,14 @@
+; Test the legalization of function signatures for RV32E.
+test legalizer
+isa riscv enable_e
+
+; regex: V=v\d+
+
+function f() {
+    ; Spilling into the stack args after %x15 since %16 and up are not
+    ; available in RV32E.
+    sig0 = signature(i64, i64, i64, i64) -> i64
+    ; check: sig0 = signature(i32 [%x10], i32 [%x11], i32 [%x12], i32 [%x13], i32 [%x14], i32 [%x15], i32 [0], i32 [4]) -> i32 [%x10], i32 [%x11]
+ebb0:
+    return
+}