esp: add support for the Espressif ESP32 chip

This is only very minimal support. More support (such as tinygo flash,
or peripheral access) should be added in later commits, to keep this one
focused.

Importantly, this commit changes the LLVM repo from llvm/llvm-project to
tinygo-org/llvm-project. This provides a little bit of versioning in
case something changes in the Espressif fork. If we want to upgrade to
LLVM 11 it's easy to switch back to llvm/llvm-project until Espressif
has updated their fork.

azure-pipelines.yml

@@ -14,20 +14,24 @@ jobs:
       inputs:
         version: '1.15'
     - checkout: self
-    - task: CacheBeta@0
+    - task: Cache@2
       displayName: Cache LLVM source
       inputs:
-        key: llvm-source-10-windows-v0
+        key: llvm-source-10-windows-v1
         path: llvm-project
     - task: Bash@3
       displayName: Download LLVM source
       inputs:
         targetType: inline
-        script: make llvm-source
+        script: |
+          make llvm-source
+          # Workaround for bad symlinks:
+          # https://github.com/microsoft/azure-pipelines-tasks/issues/13418
+          rm -f llvm-project/libcxx/test/std/input.output/filesystems/Inputs/static_test_env/bad_symlink
     - task: CacheBeta@0
       displayName: Cache LLVM build
       inputs:
-        key: llvm-build-10-windows-v0
+        key: llvm-build-10-windows-v1
         path: llvm-build
     - task: Bash@3
       displayName: Build LLVM
@@ -80,4 +84,4 @@ jobs:
         script: |
           export PATH="$PATH:./llvm-build/bin:/c/Program Files/qemu"
           unset GOROOT
-          make smoketest TINYGO=build/tinygo AVR=0
+          make smoketest TINYGO=build/tinygo AVR=0 XTENSA=0