lib: os: cbprintf: Use rodata section on sparc

Use rodata region markers to detect if string is read only on
sparc. It was previously disabled but now can be used.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
diff --git a/lib/os/cbprintf_packaged.c b/lib/os/cbprintf_packaged.c
index 577cc6d..3b18a1d 100644
--- a/lib/os/cbprintf_packaged.c
+++ b/lib/os/cbprintf_packaged.c
@@ -31,7 +31,7 @@
 #define RO_END 0
 #elif defined(CONFIG_ARC) || defined(CONFIG_ARM) || defined(CONFIG_X86) \
 	|| defined(CONFIG_RISCV) || defined(CONFIG_ARM64) \
-	|| defined(CONFIG_NIOS2) || defined(CONFIG_MIPS)
+	|| defined(CONFIG_NIOS2) || defined(CONFIG_MIPS) || defined(CONFIG_SPARC)
 	extern char __rodata_region_start[];
 	extern char __rodata_region_end[];
 #define RO_START __rodata_region_start