builtin,v.gen.wasm: support -b wasm -d no_imports (#24188)

vlib/builtin/wasm/alloc.v

@@ -12,7 +12,11 @@ __global g_heap_base = usize(vwasm_heap_base())
 @[unsafe]
 pub fn malloc(n isize) &u8 {
 	if n <= 0 {
-		panic('malloc(n <= 0)')
+		$if no_imports ? {
+			return unsafe { nil }
+		} $else {
+			panic('malloc(n <= 0)')
+		}
 	}
 
 	res := g_heap_base

vlib/builtin/wasm/builtin.v

@@ -43,9 +43,11 @@ pub fn vwasm_memory_grow(size int) int {
 @[unsafe]
 pub fn vcalloc(n isize) &u8 {
 	if n <= 0 {
-		panic('vcalloc(n <= 0)')
-	} else if n == 0 {
-		return &u8(0)
+		$if no_imports ? {
+			return unsafe { nil }
+		} $else {
+			panic('valloc(n <= 0)')
+		}
 	}
 
 	res := unsafe { malloc(n) }

vlib/v/gen/wasm/comptime.v

@@ -47,6 +47,9 @@ pub fn (mut g Gen) comptime_cond(cond ast.Expr, pkg_exists bool) bool {
 		ast.ComptimeCall {
 			return pkg_exists // more documentation needed here...
 		}
+		ast.PostfixExpr {
+			return g.comptime_if_to_ifdef((cond.expr as ast.Ident).name, true)
+		}
 		else {}
 	}
 	g.w_error('wasm.comptime_cond(): unhandled node: ' + cond.type_name())
@@ -118,6 +121,14 @@ pub fn (mut g Gen) comptime_if_to_ifdef(name string, is_comptime_option bool) bo
 			return false
 		}
 		else {
+			// note: this works but there might be some things missing from what I saw in the other platforms
+			//		 but it is better than nothing
+			if name in g.pref.compile_defines {
+				return true
+			} else {
+				return false
+			}
+
 			// taken from JS: what does it do??
 			/*
 			if is_comptime_option

vlib/v/help/build/build-wasm.txt

@@ -16,6 +16,10 @@ For more general build help, see also `v help build`.
       Overrides the default stack_top value used by the WebAssembly compiler.
       Useful for some platforms with unusual memory requirements.
 
+   -d no_imports
+      Removes the default imports from the builtins. Useful for embedded targets where
+      you can't control the imports.
+
    -os <browser|wasi>, -target-os <browser|wasi>
       Change the target WebAssembly execution environment that V compiles for.
 
@@ -25,7 +29,7 @@ For more general build help, see also `v help build`.
         WASI provides a standardized interface to interact with the host operating system,
 		allowing WebAssembly modules to perform tasks like file I/O, networking, and more.
         The specific version of the WASI specification targeted by V is 'wasi_snapshot_preview1'.
-      
+
       The `browser` target is an experimental environment that compiles for a stripped down
 	    builtin, for use in browsers. The produced WebAssembly module will have functions
 		exported that are `pub` and inside the `module main`. See `examples/wasm/mandelbrot`