Add Mac pipeline for building Mac toolchain

Doesn't build stdlib yet - you will need to copy the stdlib from the Linux build.

[skip ci] - I'll manually trigger the Mac build.

Author: zhuowei

README.md

@@ -1,3 +1,9 @@
 Build scripts for SwiftWasm.
 
+Ubuntu 16.04 build:
+
 [![Build Status](https://dev.azure.com/swiftwasm/swiftwasm-sdk/_apis/build/status/swiftwasm.swiftwasm-sdk?branchName=master)](https://dev.azure.com/swiftwasm/swiftwasm-sdk/_build/latest?definitionId=3&branchName=master)
+
+macOS 10.14 build:
+
+[![Build Status](https://dev.azure.com/swiftwasm/swiftwasm-sdk/_apis/build/status/swiftwasm.swiftwasm-sdk-mac?branchName=master)](https://dev.azure.com/swiftwasm/swiftwasm-sdk/_build/latest?definitionId=4&branchName=master)

ci-build-mac.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# On Mac, we don't build Stdlib, just the compiler, since macOS doesn't support cross compiling for non-Darwin targets.
+set -e
+sourcedir="$PWD"
+# install dependencies
+brew install ninja llvm
+# start build
+cd swift
+utils/build-script --release \
+	--llvm-targets-to-build "X86;WebAssembly" \
+	--llvm-max-parallel-lto-link-jobs 1 --swift-tools-max-parallel-lto-link-jobs 1 \
+	--stdlib-deployment-targets "macosx-x86_64" \
+	--extra-cmake-options="-DSWIFT_WASM_WASI_SDK_PATH=/usr/local/opt/llvm" \
+	--install-swift \
+	--install-prefix="/opt/swiftwasm-sdk" \
+	--install-destdir="$sourcedir/install" \
+	--installable-package="$sourcedir/swiftwasm-mac.tar.gz"
+	"$@"
+# copy the result
+cp "$sourcedir/swiftwasm-mac.tar.gz" "$BUILD_ARTIFACTSTAGINGDIRECTORY/"
+

mac-azure-pipelines.yml

@@ -0,0 +1,24 @@
+trigger:
+- master
+
+jobs:
+- job: Build
+  timeoutInMinutes: 0
+  pool:
+    vmImage: 'macos-10.14'
+  steps:
+  - script: |
+      ./ci-build-mac.sh
+    displayName: 'ci-build'
+    timeoutInMinutes: 0
+  - task: PublishBuildArtifacts@1
+    inputs:
+      pathtoPublish: $(Build.ArtifactStagingDirectory)
+      artifactName: swiftwasm-sdk-mac
+  - task: GitHubRelease@0
+    inputs:
+      gitHubConnection: github-release
+      tagSource: manual
+      tag: $(Build.BuildNumber)
+      assets: |
+        $(Build.ArtifactStagingDirectory)/*