package/gcc: allow uclibc only for gcc or1k (5.x)

uClibc doesn't build with the upstream binutils 2.32.x and gcc or1k
port due to the following error:

LD libuClibc-1.0.31.so
/opt/openrisc--uclibc--bleeding-edge-1/lib/gcc/or1k-buildroot-linux-uclibc/9.2.0/../../../../or1k-buildroot-linux-uclibc/bin/ld:
libc/libc_so.a(or1k_clone.os): pc-relative relocation against dynamic symbol
__syscall_error

See:
https://gitlab.com/kubu93/toolchains-builder/-/jobs/270854456

This error message come from a new check in binutils 2.32.x:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=f2c1801f6255a3f9f483ae2f07c7d7da0ddae4af

This issue has been reported on the uClibc-ng mailing list:
https://mailman.uclibc-ng.org/pipermail/devel/2019-August/001885.html



Since gcc 9.1 needs binutils 2.32.x or later to build successfully for
or1k, there is no binutils version left that can build gcc 9.1 and
uClibc.

For now, disable uClibc if gcc 9.1 is used for or1k.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Waldemar Brodkorb <mail@waldemar-brodkorb.de>
[Arnout: invert the logic, like in the rest of the file]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>