feat(hooks): Implement toolchains support

Introduce several toolschains for x86 and arm system. Emscripten is also
viewed as a toolchain which is impotrted this time for later JS binding
usage.

Author: yillkid

.bazelrc

@@ -43,4 +43,5 @@ build:ubsan --copt -fno-omit-frame-pointer
 build:ubsan --linkopt -fsanitize=undefined
 build:ubsan --linkopt -lubsan
 
+import %workspace%/hooks/toolchains.rc
 try-import %workspace%/user.bazelrc

WORKSPACE

@@ -7,6 +7,12 @@ git_repository(
     remote = "https://github.com/iotaledger/rules_iota.git",
 )
 
+git_repository(
+    name = "iota_toolchains",
+    commit = "0f93f76fdff9f091dd3a99aa3f19b5d3f4f7f7fc",
+    remote = "https://github.com/iotaledger/toolchains.git",
+)
+
 git_repository(
     name = "entangled",
     commit = "6ad56514a8a13fb1bf01beb10934cb9e3fd1a8a4",
@@ -33,3 +39,11 @@ iota_deps()
 third_party_deps()
 
 _cc_image_repos()
+
+load("@iota_toolchains//:toolchains.bzl", "setup_initial_deps")
+
+setup_initial_deps()
+
+load("@iota_toolchains//:defs.bzl", "setup_toolchains_repositories")
+
+setup_toolchains_repositories()

hooks/toolchains.rc

@@ -0,0 +1,20 @@
+    
+build:aarch64 --crosstool_top=@iota_toolchains//tools/aarch64--glibc--bleeding-edge-2018.07-1:toolchain
+build:aarch64 --cpu=aarch64
+build:aarch64 --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+
+build:armv7 --crosstool_top=@iota_toolchains//tools/armv7-eabihf--glibc--bleeding-edge-2018.07-1:toolchain
+build:armv7 --cpu=armeabi-v7a
+build:armv7 --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+
+build:i686 --crosstool_top=@iota_toolchains//tools/x86-i686--glibc--bleeding-edge-2018.07-1:toolchain
+build:i686 --cpu=i686
+build:i686 --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+
+build:x86_64 --crosstool_top=@iota_toolchains//tools/x86-64-core-i7--glibc--bleeding-edge-2018.07-1:toolchain
+build:x86_64 --cpu=x86_64
+build:x86_64 --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+
+build:emscripten --crosstool_top=@iota_toolchains//tools/emscripten:toolchain
+build:emscripten --cpu=emscripten
+build:emscripten --host_crosstool_top=@bazel_tools//tools/cpp:toolchain