complibs/cloog: add support for the ISL backend

CLooG 0.18+ will use ISL instead of PPL, so we have to configure
adequately depending of which backend is in use.

The Kconfig entries will decide for us which is selected, so we
can rely on either PPL xor ISL to be selected, not both.

Reported-by: "Plotnikov Dmitry" <leitz@ispras.ru>
[Dmitry did a preliminray patch to add ISL support,
which this patch is inspired from]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # MIPS specific config options

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64 if EXPERIMENTAL

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_WITH_ARCH

 ## select ARCH_SUPPORTS_WITH_TUNE

 ## select ARCH_SUPPORTS_WITH_FLOAT

 ##

 ## help The MIPS architecture, as defined by:

 ## help     http://www.mips.com/