Use unix_cc_toolchain_config from @bazel_tools (#75)

Closes #23.

Notes:
1. `cc_toolchain_config` is now a macro.
2. [Make variables](https://github.com/grailbio/bazel-toolchain/blob/f2d1ba2c9d713b2aa6e7063f6d11dd3d64aa402a/toolchain/cc_toolchain_config.bzl.tpl#L527-L537) and [Framework paths](https://github.com/grailbio/bazel-toolchain/blob/f2d1ba2c9d713b2aa6e7063f6d11dd3d64aa402a/toolchain/cc_toolchain_config.bzl.tpl#L347-L365) ([only used on macOS](https://github.com/grailbio/bazel-toolchain/blob/f2d1ba2c9d713b2aa6e7063f6d11dd3d64aa402a/toolchain/cc_toolchain_config.bzl.tpl#L498-L499)) are not yet supported; see in-code comments (beginning ## NOTE:).
3. `llvm-strip` is now used for llvm versions ≥7 as the strip binary.
4. Contains fixes for using/running the toolchain from other locations in the execroot. Most notably, `rules_foreign_cc` will use a different PWD than the execroot root for cmake targets which breaks the wrapper script. This commit has the wrapper script search alongside itself for `clang` when it isn't in the usual places.
5. Modern versions of ld.lld support start and end groups but we don't use ld.lld on macOS (yet), so don't use this feature.

Author: rrbutani

WORKSPACE

@@ -87,8 +87,6 @@ http_archive(
     urls = ["https://www.openssl.org/source/openssl-1.1.1c.tar.gz"],
 )
 
-load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-
 http_archive(
     name = "io_bazel_rules_go",
     sha256 = "8e968b5fcea1d2d64071872b12737bbb5514524ee5f0a4f54f5920266c261acb",