liblzma: Use SSE2 intrinsics instead of memcpy() in dict_repeat()

liblzma: Use SSE2 intrinsics instead of memcpy() in dict_repeat()

SSE2 is supported on every x86-64 processor. The SSE2 code is used on
32-bit x86 if compiler options permit unconditional use of SSE2.

dict_repeat() copies short random-sized unaligned buffers. At least
on glibc, FreeBSD, and Windows (MSYS2, UCRT, MSVCRT), memcpy() is
clearly faster than byte-by-byte copying in this use case. Compared
to the memcpy() version, the new SSE2 version reduces decompression
time by 0-5 % depending on the machine and libc. It should never be
slower than the memcpy() version.

However, on musl 1.2.5 on x86-64, the memcpy() version is the slowest.
Compared to the memcpy() version:

  - The byte-by-version takes 6-7 % less time to decompress.
  - The SSE2 version takes 16-18 % less time to decompress.

The numbers are from decompressing a Linux kernel source tarball in
single-threaded mode on older AMD and Intel systems. The tarball
compresses well, and thus dict_repeat() performance matters more
than with some other files.