esp8266/esp8266_common.ld: Put mp_keyboard_interrupt in iRAM.

This function may be called from a UART IRQ, which may interrupt the system
when it is erasing/reading/writing flash.  In such a case all code
executing from the IRQ must be in iRAM (because the SPI flash is busy), so
put mp_keyboard_interrupt in iRAM so ctrl-C can be caught during flash
access.

This patch also takes get_fattime out of iRAM and puts it in iROM to make
space for mp_keyboard_interrupt.  There's no real need to have get_fattime
in iRAM because it calls other functions in iROM.

Fixes issue #3897.

ports/esp8266/esp8266_common.ld

@@ -134,10 +134,15 @@ SECTIONS
         *lib/mp-readline/*.o(.literal*, .text*)
         *lib/netutils/*.o*(.literal*, .text*)
         *lib/timeutils/*.o*(.literal*, .text*)
-        *lib/utils/*.o*(.literal*, .text*)
+        *lib/utils/printf.o*(.literal*, .text*)
+        *lib/utils/sys_stdio_mphal.o*(.literal*, .text*)
+        *lib/utils/pyexec.o*(.literal*, .text*)
+        *lib/utils/stdout_helpers.o*(.literal*, .text*)
+        *lib/utils/interrupt_char.o*(.literal.mp_hal_set_interrupt_char, .text.mp_hal_set_interrupt_char)
         *drivers/bus/*.o(.literal* .text*)
 
         build/main.o(.literal* .text*)
+        *fatfs_port.o(.literal* .text*)
         *gccollect.o(.literal* .text*)
         *gchelper.o(.literal* .text*)
         *help.o(.literal* .text*)