Mesa's radeonsi gallium driver requires libelf. Add a libelf recipe that builds
only the elfutils lib/+libelf/ subdirs (avoiding the libdw/libasm/tools glibc
surface) as a PIC static archive, relying on the new relibc headers/functions
(byteswap/error/libintl/ar/search + tsearch/mempcpy/rawmemchr). Installs
libelf.a + libelf.h/gelf.h/nlist.h + libelf.pc. Wired into mesa deps.
Verified: cook libelf - successful.
