package/libopenssl: make use of linux-generic64 for 64-bit archs

package/libopenssl: make use of linux-generic64 for 64-bit archs

It was tested with:

BR2_mips64el=y
BR2_MIPS_NABI64=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-mips64-n64-full-2019.05.1.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_5=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_5_1=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
BR2_PACKAGE_OPENSSL=y

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>