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 M68k-specific values

CT_DoArchTupleValues() {

    :

}