gitBOM support via environment variable GITBOM_BUILD_MODE

[yonhan3]

Signed-off-by: Yongkui Han [email protected]

[yonhan3]

This commit adds gitBOM support for GCC compiler.
This aligns with binutils's gitBOM support.

1. support of below environmental variables to turn on gitBOM feature:
   GITBOM_BUILD_MODE=sha1,sha256,create_adg,embed_bomid,record_hash
   GITBOM_ADG_DIR=/any/directory

Each of those features can be individually turned on/off with the GITBOM_BUILD_MODE environment variable.
Refer to omnibor/spec#22 for details on the proposal.

2. support of the same symlink farm feature as Bomsh, to persist the artifact-id to ADG-doc mappings in the file system.
   all the symlinks are created in $GITBOM_ADG_DIR/symlinks/ directory.

3. when create_adg is configured, all the ADG docs are created in $GITBOM_ADG_DIR/objects/ directory, and the symlink for the output file is created.

4. when embed_bomid is configured, then create_adg feature is also enabled.
   all the ADG docs are created in $GITBOM_ADG_DIR/objects/ directory, and the symlink for the output file is created.
   and correct bom-ids are embedded in the output ELF file.

5. the new .note.gitbom NOTE ELF section format is used, with NT_GITBOM_SHA1=1, and NT_GITBOM_SHA256=2.
   the binutils' readelf tool has been enhanced to recognize the new .note.gitbom section.

6. when record_hash is enabled, it records the hashes of output/input files in the $GITBOM_ADG_DIR/metadata/gcc/*.gitbom_raw_logfile,
   in the same format as Bomsh's /tmp/bomsh_hook_raw_logfile file, so Bomsh scripts can process these *.gitbom_raw_logfile directly.

7. The combinations of sha1-only, sha256-only, or sha1+sha256 are all supported.

8. all gcc compilations are supported, even with non-ELF files as output file, like:
   gcc -S -o hello.s hello.c
   gcc -c -o hello.o hello.s
   gcc -o hello hello.o
   gcc -S -o hello2.s hello2.c
   gcc -o hello2 hello2.s myfunc2.c libhello.a

9. "unset GITBOM_BUILD_MODE" or "GITBOM_BUILD_MODE=" will turn off gitBOM support.