Fix build with modern GCC (#933)

Fix build with modern GCC (#933)

* GCC: MSP430F449: Add missing attributes

Apparently at some point in the past, GCC (or TI's GCC) used to define
these attributes. Define them ourselves so that we can compile the demo
application.

* GCC: MSP430F449: Make interrupts return void

If a return type of a function is not specified, it defaults to int. Set
the return type of interrupts to void to avoid warnings.

* GCC: MSP430F449: Define portPOINTER_SIZE_TYPE

portPOINTER_SIZE_TYPE defaults to uint32_t if undefined. Define it to
uint16_t, which is correct for this port.