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>

soc/riscv/openisa_rv32m1/linker.ld

@@ -133,6 +133,12 @@ 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.*)
 	*(.eh_frame)