[code] add __SRAM section switch, allowing sram functions

-On a twosegment system (like all of the cortex-m*) systems, this
allows you to declare a function that lives in sram. On all the others
it has essentially no effect.

arch/arm/system-onesegment.ld

@@ -10,7 +10,7 @@ SECTIONS
 	.text.boot.vectab1 : { KEEP(*(.text.boot.vectab1)) }
 	.text.boot.vectab2 : { KEEP(*(.text.boot.vectab2)) }
 	.text.boot : { KEEP(*(.text.boot)) }
-	.text :	{ *(.text .text.* .glue_7* .gnu.linkonce.t.*) } =0x9090
+	.text :	{ *(.text .text.* .sram.text .glue_7* .gnu.linkonce.t.*) } =0x9090
 
 	.interp : { *(.interp) }
 	.hash : { *(.hash) }