package/erlang: risc-v should only add risc-v

package/erlang: risc-v should only add risc-v

This restricts the Erlang RISC-V update from
0bab8c1089983a4b5f0f2acfe5cad77b02d6f654 to only enabling RISCV_64
rather than enabling builds on all platforms with toolchains that
include atomics.

This fixes build errors on SPARCv8 and other platforms that look like
this:

/tmp/ccD9Q9F7.s: Assembler messages:
/tmp/ccD9Q9F7.s:230: Error: Architecture mismatch on "cas [%g3],%g2,%g1".
/tmp/ccD9Q9F7.s:230: (Requires v9|v9a|v9b|v9c|v9d|v9e|v9v|v9m|m8; requested architecture is v8.)

Fixes:
http://autobuild.buildroot.net/results/a57/a5779e22c41eb63b8400409f26f96b924fdde1bc/

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>