update-linux-headers: fix forwarding to asm-generic headers

update-linux-headers: fix forwarding to asm-generic headers

Afer commit 3efc75ad9d9 ("scripts/update-linux-headers.sh: Remove
temporary directory inbetween", 2024-05-29), updating linux-headers/
results in errors such as

   cp: cannot stat '/tmp/tmp.1A1Eejh1UE/headers/include/asm/bitsperlong.h': No such file or directory

because Loongarch does not have an asm/bitsperlong.h file and uses the
generic version.  Before commit 3efc75ad9d9, the missing file would
incorrectly cause stale files to be included in linux-headers/.  The files
were never committed to qemu.git, but were wrong nevertheless. The build
would just use the system version of the files, which is opposite to
the idea of importing Linux header files into QEMU's tree.

Create forwarding headers, resembling the ones that are generated during a
kernel build by scripts/Makefile.asm-generic, if a file is only installed
under include/asm-generic/.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>