Fix generator polyglot bootstrap script failures on Windows and macOS

[solevic]

Problem

The current stub.sh generator script fails to run correctly on certain platforms:

• Windows: Requires CRLF line endings for the Batch portion of the polyglot script
• macOS: Newer versions require a valid shebang

Context

The original polyglot script was generated via sbt-assembly's defaultUniversalScript, which handled mixed line endings and shebang injection.

Having a stub.sh with mixed line endings is not possible because of Git line endings normalization.

Proposed Solution

The stub should be generated dynamically during the build process using a Bazel genrule. This allows to explicitly write CRLF bytes for the Windows section while keeping LF for the Unix section.

1. Create a stub genrule: This rule constructs the script line-by-line, enforcing the correct line endings.

genrule(
    name = "stub",
    outs = ["stub.sh"],
    cmd = """
    printf '#!/usr/bin/env sh\n' > $@
    printf '@ 2>/dev/null # 2>nul & echo off & goto BOF\n' >> $@
    printf ':\n' >> $@
    printf 'exec java -jar $${JAVA_OPTS:-} "$$0" "$$@"\n' >> $@
    printf 'exit\n' >> $@
    printf '\n' >> $@
    printf ':BOF\r\n' >> $@
    printf '@echo off\r\n' >> $@
    printf 'java -jar %%JAVA_OPTS%% "%%~dpnx0" %%*\r\n' >> $@
    printf 'exit /B %%errorlevel%%\r\n' >> $@
    """,
)

2. Update the generator rule: Depend on the generated stub and concatenate it with the deploy jar.

genrule(
    name = "generator",
    srcs = [":stub", "//src:djinni_deploy.jar"],
    outs = ["djinni"],
    cmd = "cat $(location :stub) $(location //src:djinni_deploy.jar) > $@ && chmod +x $@",
    executable = True
)

If this solution is appropriate, I can provide a pull request with the changes.