toolchain: add uClibc 0.9.31, mark 0.9.30.x as recent

Enable susv3/susv4 legacy support for now, as a lot of packages (E.G.
busybox) breaks with the stricter interpretation in 0.9.31.
Also slightly tweak uclibc.mk as the "new" linuxthreads symbol changed.

Test built on x86/x86-64/ppc/arm/mips.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>