all: make WebAssembly initial linear memory size configurable

When the target supports it, allow the (initial) heap size to be
configured. Currently only supported in WebAssembly.

This also changes the default heap size of WebAssembly from 64kB to 1MB.

src/runtime/arch_wasm.go

@@ -14,9 +14,12 @@ const TargetBits = 32
 //go:extern __heap_base
 var heapStartSymbol unsafe.Pointer
 
+//go:export llvm.wasm.memory.size.i32
+func wasm_memory_size(index int32) int32
+
 var (
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
-	heapEnd   = (heapStart + wasmPageSize - 1) &^ (wasmPageSize - 1) // conservative guess: one page of heap memory
+	heapEnd   = uintptr(wasm_memory_size(0) * wasmPageSize)
 )
 
 const wasmPageSize = 64 * 1024