kernel: adjust default main stack for riscv

We are just at the knife edge with 512, with stack
overflows being observed with stack canaries enabled.
Given the special case for the idle thread stack size
on this arch, seems reasonable to increase it here
for that arch.

Signed-off-by: Andrew Boie <[email protected]>

Author: Andrew Boie

kernel/Kconfig

@@ -148,7 +148,7 @@ config SCHED_CPU_MASK
 config MAIN_STACK_SIZE
 	int "Size of stack for initialization and main thread"
 	default 2048 if COVERAGE_GCOV
-	default 512 if ZTEST
+	default 512 if ZTEST && !RISCV32
 	default 1024
 	help
 	  When the initialization is complete, the thread executing it then