tag default_interrupt_handler as function

[nerdralph]

The vector table entries need the low bit set for Thumb-2 code. Section 2.3.4 of the Programming Manual states: "The least-sigificant bit of each vector must be 1, indicating that the exception handler is Thumb code." The fix is to add: .type default_interrupt_handler, %function so the assembler/linker knows to set the low bit.