Makefile: introduce PROG_NAME to contain the 'ct-ng' executable name

With the upcomming --program-{prefix,suffix,transform-name} options,
we'll have a need to centralise the actual executable name, that is
now no longer be a constant.

Rather than spread the prefix/suffix through-out the code, just
centralise the name setting in one place. Beside, transform-name
would not be possible without setting the name at a single location.

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

# Compute AVR32-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

    # gcc ./configure flags

    CT_ARCH_WITH_ARCH=

    CT_ARCH_WITH_ABI=

    CT_ARCH_WITH_CPU=

    CT_ARCH_WITH_TUNE=

    CT_ARCH_WITH_FPU=

    CT_ARCH_WITH_FLOAT=

    CT_TARGET_SYS=none

}