riscv: Fix guest/init linkage for multilib toolchain

For RISC-V multilib toolchains, we must specify -mabi and -march
options when linking guest/init.

Fixes: 2e996783 ("riscv: Initial skeletal support")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231118132847.758785-7-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>