[PATCH] libbpf: Define __WORDSIZE if not available

[Arnaldo Carvalho de Melo]

Some systems, such as Android, don't have a define for __WORDSIZE, do it
in terms of __SIZEOF_LONG__, as done in perf since 2012:

  http://git.kernel.org/torvalds/c/3f34f6c0233ae055b5

For reference: https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

I build tested it here and Andrii did some Travis CI build tests too.

Acked-by: Andrii Nakryiko <andriin@xxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>