Publish a CIPD symbols package for ELF binaries

A symbols package is used for symbolization in logs and debugging crashes. A CIPD prebuilt package that contains Fuchsia ELF binaries needs to have a companion symbols package that contains debug information for the binaries.

Requirements for a symbols package

Each ELF binary in the original CIPD package must include an ELF NT_GNU_BUILD_ID note section, which includes a unique build-id hash value that uniquely identifies its non-debug-related content.

This note is created at link time when producing the ELF binary. Recent versions of GCC, or the prebuilt Fuchsia Clang toolchain, produce it automatically. However, regular Clang requires passing a special linker flag (that is, -Wl,--build-id).

Note: To print the build-id of an existing library, use either one of file <LIBRARY> or readelf -n <LIBRARY> | grep "Build ID"
The symbols package must use the directory layout typical of .build-id directories used to store debug information in separate files.

This means that each file must be stored as <xx>/<xxxxxxxxxx>.debug, where <xx> and <xxxxxxxxx> are hex strings derived from the build-id hash value, of the unstripped binary. Each such file should match one stripped ELF binary with the same build-id from the original package.

Note: The symbols package needs to not include the top-level .build-id directory.

This example shows the directory structure of a CIPD symbols package with unstripped ELF binaries:
```
1d/
  bca0bd1be33e19.debug
1f/
  512abdcbe453ee.debug
  90dd45623deab1.debug
2b/
  0e519bcf3942dd.debug
3d/
  aca0b11beff127.debug
5b/
  66bc85af2da641697328996cbc04d62b84fc58.debug
```
The symbols package must use the same version identifiers (tags) as the original CIPD package they refer to. This allows them to be rolled together.
If several CIPD packages that contain stripped ELF binaries are rolled together (using the same version identifiers), then grouping the debug symbols for all of them in a single CIPD symbols package is acceptable, but not required.
The CIPD path for the symbols package needs to use the the following suffix:
```
-debug-symbols-<ARCH>
```
For example, myproject/fuchsia/mypackage-debug-symbols-amd64 contains the symbols for the myproject/fuchsia/mypackage-amd64 prebuilt package.
The Jiri checkout path for all symbols package must be ${FUCHSIA_DIR}/prebuilt/.build-id.

Generate a symbols package

To generate a symbols package, you need to:

Compile all your ELF binaries with DWARF debug information (for example, passing the -g flag to the compiler, even in release mode).
Produce an NT_GNU_BUILD_ID note for the ELF binaries.

This is the default on recent GCC versions and the Fuchsia prebuilt Clang toolchain, but regular Clang requires passing a special flag (-Wl,--build-id) to the linker.

You can generate the stripped binary and the corresponding build-id directories with a single call to llvm-objcopy, as in the following example:

# Copy out/libfoo.so to symbols/<xx>/<xxxxxxx>.debug according to its
# `build-id` value (requires the library to be linked with -Wl,--build-id when
# using Clang), and also copy the stripped version of the library to
# stripped/libfoo.so
#
# NOTE: To strip executables, instead of libraries, replace --strip-all below
#       with --strip-sections
#
UNSTRIPPED_LIB=out/libfoo.so
STRIPPED_LIB=stripped/libfoo.so
SYMBOLS_DIR=./symbols

llvm-objcopy --strip-all \
    --build-id-link-dir="${SYMBOLS_DIR}" \
    --build-id-link-input=.debug \
    "${UNSTRIPPED_LIB}" "${STRIPPED_LIB}"

Repeat as many times as necessary to populate the symbols/ directory, then upload its content (the files under symbols/) as your symbols package.

Don't forget to copy the content of the stripped/ directory to your prebuilt CIPD package as well.