kernel: set DYNAMIC_THREAD_STACK_SIZE to 4096 for coverage

Increase stack sizes to allow coverage to complete.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/Kconfig b/kernel/Kconfig
index bb2c5ba..447f124 100644
--- a/kernel/Kconfig
+++ b/kernel/Kconfig
@@ -246,7 +246,7 @@
 
 config DYNAMIC_THREAD_STACK_SIZE
 	int "Size of each pre-allocated thread stack"
-	default 4096 if X86
+	default 4096 if X86 || COVERAGE_GCOV
 	default 1024 if !X86 && !64BIT
 	default 2048 if !X86 && 64BIT
 	help