oniro-linux.conf: Make TCLIBC weak assignment

Re-allow overriding of TCLIBC via local.conf/shell env.
Upstream part has been merged into kirkstone branch:
oe-core:e30ef926e440b505e93e404c056034d86364a241

Fixes: #608

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>