toolchain/toolchain-buildroot: enable uclibc for riscv64

We can enable uclibc for RISC-V 64 bit now that it has been
bumped from v1.0.32 to v1.0.34.

Uclibc has had basic support for RISC-V 64 bit since v1.0.31, but
shared library and TLS/NPTL support has only been available since
v1.0.33.

This update has been tested using qemu_riscv64_virt_defconfig and
the Buildroot host QEMU.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>