libvecfp: Remove internal dynamic allocation

This patch changes how libvecfp internally allocates the temporary
buffers by using samll stack allocations instead rely on heap memory.
libvecpf: Change make clean to remove only objects
libvecpf: Enable VSX support by default

This patch removes the '--enable-vsx' option and enables VSX '%vv'
option by default.  Since internally libvecpf just need altivec
instruction to load the vector registers and there both VSX and
altivec register interposes, there is no need to build VSX support
conditionally.
The VSX tests, however, are build conditionally iff compiler/flags
defines __VSX__ (since vector double is only support in with this
configuration).