riscv: Introduce RISCV_ALWAYS_SWITCH_THROUGH_ECALL

Some early RISC-V SoCs have a problem when an `mret` instruction is used
outside a trap handler.

After the latest Zephyr RISC-V huge rework, the arch_switch code is
indeed calling `mret` when not in handler mode, breaking some early
RISC-V platforms.

Optionally restore the old behavior by adding a new
CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL symbol.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Carlo Caione <ccaione@baylibre.com>