toolchain/toolchain-buildroot: only riscv64 is supported by uClibc-ng

The commit [1] enabled riscv32 and riscv64 for uClibc-ng
internal toolchain backend but only riscv64 is curently
supported by uClibc-ng.

The initial patch [2] from Mark Corbin is only about riscv64.

Remove riscv32 from uClibc-ng supported architecture list.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981656



[1] 209a0824
[2] bd9810e1

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>