riscv: use 16-byte alignment everywhere

Source: https://riscv.org/wp-content/uploads/2015/01/riscv-calling.pdf

src/runtime/arch_tinygoriscv64.go

@@ -6,8 +6,3 @@ const GOARCH = "arm64" // riscv pretends to be arm
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 64
-
-// Align on word boundary.
-func align(ptr uintptr) uintptr {
-	return (ptr + 7) &^ 7
-}