riscv: add initjmp()

Implement initjmp() for RISC-V, a non-standard extension to setjmp()/
longjmp() allowing to initialize a jump buffer with a function pointer
and a stack pointer. This will be useful to later introduce threads.
With this new function it becomes possible to longjmp() to a particular
function pointer (rather than to a point previously reached during
program execution as is the case with setjmp()), and with a custom stack.
Both things are needed to spin off a new thread. Then the usual
setjmp()/longjmp() pair is enough to save and restore a context, i.e.,
switch thread.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

arch/Kconfig

@@ -153,6 +153,7 @@ config RISCV
 	bool "RISC-V architecture"
 	select CREATE_ARCH_SYMLINK
 	select HAVE_SETJMP
+	select HAVE_INITJMP
 	select SUPPORT_ACPI
 	select SUPPORT_LITTLE_ENDIAN
 	select SUPPORT_OF_CONTROL

arch/riscv/lib/setjmp.S

@@ -59,3 +59,14 @@ ENTRY(longjmp)
 	ret
 ENDPROC(longjmp)
 .popsection
+
+.pushsection .text.initjmp, "ax"
+ENTRY(initjmp)
+	/* a1: entry point address, a2: stack base, a3: stack size */
+	add a2, a2, a3
+	STORE_IDX(a1, 12)
+	STORE_IDX(a2, 13)
+	li  a0, 0
+	ret
+ENDPROC(initjmp)
+.popsection