gcc: Compile without ZSTD

gcc: Compile without ZSTD

GCC can use ZSTD to compress debugging/LTO information in binary
objects. However, on riscv64, compiling zstd requires libatomic which is
not available at this point.

In order to make the build work, we explicitely disable ZSTD in GCC and
build ZSTD after libatomic is available.

Although ZSTD offers great compression, we won't have any disadvantages
through this change since we do not ship any debugging information and
at this point in time to not use LTO.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>