package/libopenssl: fix target architecture for MIPS64n32

package/libopenssl: fix target architecture for MIPS64n32

Commit 1ebb35ee5fb7bd5e6278a84ac2c18aa0a38056fb changed the libopenssl
target architecture to 'linux-generic64' for 64-bit archs based on
BR2_ARCH_IS_64. However, MIPS64n32 has BR2_ARCH_IS_64 set, but is a 32-bit
ABI. On such board, libopenssl needs to be configured with linux-generic32
to function properly.

One symptom of this problem is that ssh-keygen hangs on key generation,
waiting for more random bits. See [1] for the discussion with openssl
upstream.

Thanks to Ronny Meeus for investigating the issue and kudos to the openssl
community for their responsive and helpful interaction!

Reported-by: Ronny Meeus <ronny.meeus@nokia.com>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[1] https://mta.openssl.org/pipermail/openssl-users/2020-June/012565.html

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>