lz4toasm should now more easily accept LZ4 files output by
compressor programs that do not support the extracted-size
field, such as Stephan Brumme's smallz4.
This patch also proposes to add a new lz4len() function to
the libc: it parses an LZ4 compressed block to compute the
unpacked content size, without really unpacking the block.
Co-authored-by: tkchia <tkchia-cosmo@gmx.com>
