gcc: fix gcc version dependencies for the PowerPC64/musl exclusion

gcc: fix gcc version dependencies for the PowerPC64/musl exclusion

In commit
5ab751ca44e9da6f406876ff582b3a813056b0d4 ("toolchain-buildroot: allow to
build ppc64(le) musl toolchains"), support for building a musl toolchain
for ppc64(le) was added. Since this support only works with gcc 6, some
additional dependencies have been added to the older gcc versions so
that they cannot be selected on ppc64(le)/musl.

Unfortunately, the expression of the dependency was wrong, and leads to
those older gcc versions being non-selectable if you're not using
musl. Indeed, the dependencies look like this:

  depends on !BR2_powerpc64 && !BR2_powerpc64le && BR2_TOOLCHAIN_USES_MUSL

So as soon as you're not using musl, BR2_TOOLCHAIN_USES_MUSL is false,
so the entire condition is false, and the gcc version is not available.

Due to this, only gcc 6.x can be selected currently with uclibc or
glibc, which is clearly not the intended behavior.

This commit reworks those dependencies to:

  depends on !(BR2_TOOLCHAIN_USES_MUSL && (BR2_powerpc64 || BR2_powerpc64el))

which more clearly expresses what we want:

 "We don't want to (have a toolchain that uses musl and (be building
  either for PPC64 or PPC64le))"

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>