build/internals.sh: Replace nonportable call to stat

The call to stat to find out if a file is a symlink works only on GNU systems,
and the replacing portable call to readlink is also shorter and more concise code.

yann.morin.1998@anciens.enib.fr:
Apply simpler test, after discussion with author and Arnaud LACOMBE on the ML.

# 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

}