arch: added support for the riscv32 architecture

RISC-V is an open-source instruction set architecture.
Added support for the 32bit version of RISC-V to Zephyr.

1) exceptions/interrupts/faults are handled at the architecture
   level via the __irq_wrapper handler. Context saving/restoring
   of registers can be handled at both architecture and SOC levels.
   If SOC-specific registers need to be saved, SOC level needs to
   provide __soc_save_context and __soc_restore_context functions
   that shall be accounted by the architecture level, when
   corresponding config variable RISCV_SOC_CONTEXT_SAVE is set.

2) As RISC-V architecture does not provide a clear ISA specification
   about interrupt handling, each RISC-V SOC handles it in its own
   way. Hence, at the architecture level, the __irq_wrapper handler
   expects the following functions to be provided by the SOC level:
   __soc_is_irq: to check if the exception is the result of an
                 interrupt or not.
   __soc_handle_irq: handle pending IRQ at SOC level (ex: clear
                     pending IRQ in SOC-specific IRQ register)

3) Thread/task scheduling, as well as IRQ offloading are handled via
   the RISC-V system call ("ecall"), which is also handled via the
   __irq_wrapper handler. The _Swap asm function just calls "ecall"
   to generate an exception.

4) As there is no conventional way of handling CPU power save in
   RISC-V, the default nano_cpu_idle and nano_cpu_atomic_idle
   functions just unlock interrupts and return to the caller, without
   issuing any CPU power saving instruction. Nonetheless, to allow
   SOC-level to implement proper CPU power save, nano_cpu_idle and
   nano_cpu_atomic_idle functions are defined as __weak
   at the architecture level.

Change-Id: I980a161d0009f3f404ad22b226a6229fbb492389
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>

include/linker-defs.h

@@ -41,6 +41,8 @@
 /* Nothing yet to include */
 #elif defined(CONFIG_NIOS2)
 /* Nothing yet to include */
+#elif defined(CONFIG_RISCV32)
+/* Nothing yet to include */
 #else
 #error Arch not supported.
 #endif