toolchain/toolchain-buildroot: allow uclibc-ng for riscv

uclibc-ng supports the RISC-V architecture since version 1.0.31, so
let's allow selecting this C library when RISC-V is used.

There was a previous attempt in commit
bd9810e1, which was reverted in
e7d631c0, due to uClibc-ng not
implementing the __riscv_flush_icache() which is needed by
gcc. However this function has been implemented in upstream uClibc-ng
as of 1.0.35, so we can now safely re-enable uClibc-ng on RISC-V.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>