basic/architecture: Properly set LIB_ARCH_TUPLE for powerpcspe

On powerpc, there are two possible ABIs and hence values for
LIB_ARCH_TUPLE. The convential type with an FPU and the embedded
variant, called powerpcspe, which does not have a convential FPU
but a special Signal Processing Engine (SPE). The latter can be
detected by checking whether __NO_FPRS__ is defined.