nolibc is NOT under GPL. See first line of each file.

/* SPDX-License-Identifier: LGPL-2.1 OR MIT */

It's technically not part Linux's headers either. It's published under the tools subdirectory, so it's something that ships along with the kernel, but not used by the kernel itself. Basically it's there as some people might find it useful, but could've as well been a separate repo.