compiler, runtime: optimize zero-sized allocations

Instead of referring to an unused global, use a constant value. This is
safe even when using `-gc=none` (since no actual memory gets allocated)
which wasn't the case before. It should also reduce binary size by a few
bytes for most programs.

src/runtime/arch_tinygoriscv.go

@@ -4,6 +4,8 @@ package runtime
 
 import "device/riscv"
 
+const zeroSizeAllocPtr uintptr = 0xffff_fff0 // should be unused on most RISC-V chips
+
 const deferExtraRegs = 0
 
 const callInstSize = 4 // 8 without relaxation, maybe 4 with relaxation