Add FLASHMEM, for assigning functions (not variables) to flash

keywords.txt

 attachRts	KEYWORD2
 attachCts	KEYWORD2
 PROGMEM	LITERAL1
+FLASHMEM	LITERAL1
 DMAMEM	LITERAL1
 FASTRUN	LITERAL1
 Serial4	KEYWORD1

teensy4/avr/pgmspace.h

 #define DMAMEM __attribute__ ((section(".dmabuffers"), used))
 #define FASTRUN __attribute__ ((section(".fastrun") ))
 #define PROGMEM __attribute__((section(".progmem")))
+#define FLASHMEM __attribute__((section(".flashmem")))
 
 #define PGM_P  const char *
 #define PSTR(str) ({static const char data[] PROGMEM = (str); &data[0];})

teensy4/imxrt1062.ld

 		KEEP(*(.bootdata))
 		KEEP(*(.vectors))
 		KEEP(*(.startup))
+		*(.flashmem*)
 		*(.progmem*)
                 . = ALIGN(4);
                 KEEP(*(.init))