tools/nolibc/string: export strlen()

As with commit 8d304a37, "tools/nolibc/string: export memset() and
memmove()", gcc -Os without -ffreestanding may fail to compile with:

	cc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib -lgcc  -static -o test test.c
	/usr/bin/ld: /tmp/cccIasKL.o: in function `main':
	test.c:(.text.startup+0x1e): undefined reference to `strlen'
	collect2: error: ld returned 1 exit status

As on the aforementioned commit, this patch adds a section to export
this function so compilation works on those cases too.

Signed-off-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>