cc/gcc: pass the install prefix to the core passes

Currently, the discrimination on the core compilers prefixes depends on
the type of core compiler to build.

This is not correct, and the caller of the core backend should specify
the prefix.

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

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =