support WASI target (#1373)

* initial commit for WASI support

* merge "time" package with wasi build tag
* override syscall package with wasi build tag
* create runtime_wasm_{js,wasi}.go files
* create syscall_wasi.go file
* create time/zoneinfo_wasi.go file as the replacement of zoneinfo_js.go
* add targets/wasi.json target

* set visbility hidden for runtime extern variables

Accodring to the WASI docs (https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md#current-unstable-abi),
none of exports of WASI executable(Command) should no be accessed.

v0.19.0 of bytecodealliance/wasmetime, which is often refered to as the reference implementation of WASI,
does not accept any exports except functions and the only limited variables like "table", "memory".

* merge syscall_{baremetal,wasi}.go

* fix js target build

* mv wasi functions to syscall/wasi && implement sleepTicks

* WASI: set visibility hidden for globals variables

* mv back syscall/wasi/* to runtime package

* WASI: add test

* unexport wasi types

* WASI test: fix wasmtime path

* stop changing visibility of runtime.alloc

* use GOOS=linux, GOARCH=arm for wasi target

Signed-off-by: mathetake <takeshi@tetrate.io>

* WASI: fix build tags for os/runtime packages

Signed-off-by: mathetake <takeshi@tetrate.io>

* run WASI test only on Linux

Signed-off-by: mathetake <takeshi@tetrate.io>

* set InternalLinkage instead of changing visibility

Signed-off-by: mathetake <takeshi@tetrate.io>

src/syscall/syscall_baremetal.go

@@ -1,4 +1,4 @@
-// +build baremetal
+// +build baremetal wasi
 
 package syscall
 

src/syscall/tables_baremetal.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build baremetal nintendoswitch
+// +build baremetal nintendoswitch wasi
 
 package syscall