On x86 machines where the compiler supports -m32, use 'linux-x86'

The rationale is that the linux-x86 is the most likely config target
to evolve and should therefore be chosen when possible, while
linux-elf is mostly reserved for older Linux machines.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1924)