riscv: Enable multilib support

Turn on multilib to support the various risc-v isa options.

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

configs/riscv32.config

@@ -7,7 +7,7 @@ CT_PATCH_BUNDLED_LOCAL=y
 CT_LOCAL_PATCH_DIR="${CT_TOP_DIR}/../../patches"
 # CT_LOG_PROGRESS_BAR is not set
 CT_ARCH_RISCV=y
-# CT_DEMULTILIB is not set
+CT_MULTILIB=y
 CT_ARCH_ARCH="rv32ima"
 CT_ARCH_ABI="ilp32"
 CT_TARGET_VENDOR="zephyr"