Make sure crt0 files come first

BIOS jumps on boot to the beginning of main_ram.
Unless `_start` function of loaded binary is there
it won't work correctly.

litex/soc/software/demo/linker.ld

@@ -10,6 +10,12 @@ SECTIONS
 	.text :
 	{
 		_ftext = .;
+		/* Make sure crt0 files come first, and they, and the isr */
+		/* don't get disposed of by greedy optimisation */
+		*crt0*(.text)
+		KEEP(*crt0*(.text))
+		KEEP(*(.text.isr))
+
 		*(.text .stub .text.* .gnu.linkonce.t.*)
 		_etext = .;
 	} > main_ram