powerpc/64: Add big-endian ELFv2 flavour to crypto VMX asm generation

This allows asm generation for big-endian ELFv2 builds.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221128041539.1742489-4-npiggin@gmail.com