stdlib: add support for WebAssembly in VarArgs (#39361)

When targeting `wasm32` we're relying on the alignment padding branch that's used for 32-bit ARM.

Author: MaxDesiatov

stdlib/public/core/VarArgs.swift

@@ -601,7 +601,7 @@ final internal class __VaListBuilder {
     // supported vararg type is greater than the alignment of Int, such
     // as non-iOS ARM. Note that we can't use alignof because it
     // differs from ABI alignment on some architectures.
-#if (arch(arm) && !os(iOS)) || arch(arm64_32)
+#if (arch(arm) && !os(iOS)) || arch(arm64_32) || arch(wasm32)
     if let arg = arg as? _CVarArgAligned {
       let alignmentInWords = arg._cVarArgAlignment / MemoryLayout<Int>.size
       let misalignmentInWords = count % alignmentInWords