kernel-build.eclass: Force GNU tar for kernel builds

Override the tar executable used by kernel Makefiles to force GNU tar.
This is necessary since some of the scripts use options specific
to GNU tar, and therefore fail with other choices for
app-alternatives/tar.  That said, this override works only with the most
recent version of sys-kernel/gentoo-kernel.  It doesn't cover all
upstream uses of tar, but it does cover these that fail with other tar
implementations.

Closes: https://bugs.gentoo.org/951102
Signed-off-by: Michał Górny <mgorny@gentoo.org>

eclass/kernel-build.eclass

@@ -47,6 +47,7 @@ BDEPEND="
 	${PYTHON_DEPS}
 	app-alternatives/cpio
 	app-alternatives/bc
+	app-arch/tar
 	dev-lang/perl
 	sys-devel/bison
 	sys-devel/flex
@@ -226,6 +227,7 @@ kernel-build_src_configure() {
 		OBJCOPY="$(tc-getOBJCOPY)"
 		OBJDUMP="$(tc-getOBJDUMP)"
 		READELF="$(tc-getREADELF)"
+		TAR=gtar
 
 		# we need to pass it to override colliding Gentoo envvar
 		ARCH=$(tc-arch-kernel)