debug/gdb: companion libs are not used

Although the gdb ./configure advertises for GMP and MPFR, those libraries
are not used by gdb (the ./configure is used across different packages,
hence the check for GMP/MPFR). See:
http://sourceware.org/ml/crossgcc/2010-08/msg00168.html

The same applies to MPC.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }