gmp: fix EXTRA_OECONF for mipsarchr6

previously used mipsarchr6:append created an empty set for
EXTRA_OECONF and then appended --disable-assembly
while it should be just added to the existing var value.
Without this patch gmp will be configured without
--enable-cxx=detect for mipsarchr6 targets

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>