IMPORT: xz: import the xz-embedded decoder

IMPORT: xz: import the xz-embedded decoder

Import the .xz decoder from xz-embedded v2024-12-30 (0BSD). Only what is
needed for a plain LZMA2 stream is taken: the stream and LZMA2 decoders and
the two check modules; BCJ filters, SHA-256 and the kernel glue are left
out. The Makefile enables CRC64, the integrity check "xz" uses by default,
and links the decoder only when the compiler produces ELF objects, since
its only user in sym.c is not compiled for another object format.

Files are imported verbatim for later resyncing: C files in src/, headers
in include/import/. As the C files include their private headers quoted,
src/ carries small wrappers including the real ones, as src/cebtree-prv.h
does. include/import/xz_config.h maps the few kernel calls onto malloc()
and string functions.