runtime: use 32-bit function index on Wasm

Following CL 567896, this is one more place we used only 16 bits
for the function index. Change it to load 32 bits.

For #64856.

Change-Id: I66a78c086e67165604053313751c097a70c50ba9
Reviewed-on: https://go-review.googlesource.com/c/go/+/609118
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: cherrymui

src/runtime/asm_wasm.s

@@ -532,7 +532,7 @@ TEXT wasm_pc_f_loop(SB),NOSPLIT,$0
 			Get SP
 			I32Const $8
 			I32Sub
-			I32Load16U $2 // PC_F
+			I32Load $2 // PC_F
 
 			CallIndirect $0
 			Drop
@@ -568,7 +568,7 @@ outer:
 			Get SP
 			I32Const $8
 			I32Sub
-			I32Load16U $2 // PC_F
+			I32Load $2 // PC_F
 			Tee R2
 
 			Get R0