cmake: Add the TEXT_START location to zephyr_linker_sources()

Places linker code at or near the beginning of the text section.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>

include/arch/riscv/common/linker.ld

@@ -88,6 +88,12 @@ SECTIONS
 		KEEP(*(".openocd_debug.*"))
 
 		_image_text_start = .;
+
+/* Located in generated directory. This file is populated by the
+ * zephyr_linker_sources() Cmake function.
+ */
+#include <snippets-text-start.ld>
+
 		*(.text)
 		*(".text.*")
 		*(.gnu.linkonce.t.*)