Replace git_sha_rewriter with Bazel native approach

[htuch]

Bazel lost its forced md5 stamping a while back, so we should be able to use a genrule and -Wl,@$(location :label_for_the_file) in linkopts to replace the https://github.com/envoyproxy/envoy/blob/master/tools/git_sha_rewriter.py hack.

[htuch]

See also bazelbuild/bazel#2805.

[htuch]

See also https://docs.bazel.build/versions/master/user-manual.html#workspace_status (improved docs on workspace status command) and related documentation issue bazelbuild/bazel#2893.

[oquenchil]

Hey,

here's a full example of the workaround while we do the cleanup to support a slightly better way of passing files with options to the linker.
------------------------------BUILD file------------------------------

sh_binary(
    name = "build_id_writer",
    srcs = ["build_id_writer.sh"],
)

genrule(
    name = "build_id_flag_generator",
    outs = ["linkopts.lds"],
    stamp = 1,
    cmd = "$(location build_id_writer) > $@",
    tools = ["build_id_writer"],
)

cc_binary(
    name = "bin",
    srcs = ["main.cc"],
    linkopts = ["-Wl,@$(location linkopts.lds)"],
    deps = ["linkopts.lds"],
)

------------------------------build_id_writer.sh------------------------------

#!/bin/bash

BUILD_ID_OPTION="--build-id=0x"
GIT_COMMIT_HASH=$(grep STABLE_GIT_COMMIT bazel-out/stable-status.txt| cut -f2 -d" ")
echo "$BUILD_ID_OPTION""$GIT_COMMIT_HASH"

------------------------------workspace_status.sh------------------------------

#!/bin/bash
echo "CURRENT_TIME $(date +%s)"
echo "RANDOM_HASH $(cat /dev/urandom | head -c16 | md5sum 2>/dev/null | cut -f1 -d' ')"
echo "STABLE_GIT_COMMIT $(git rev-parse HEAD)"
echo "STABLE_USER_NAME $USER"

---

The bazel command line to build the binary would be:
bazel build --workspace_status_command=./workspace_status.sh :bin