Adding files for Microchip's PIC16 target.

Adding files for Microchip's PIC16 target.

A brief description about PIC16:
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PIC16 is an 8-bit microcontroller with only one 8-bit register which is the
accumulator. All arithmetic/load/store operations are 8-bit only.
The architecture has two address spaces: program and data. The program memory
is divided into 2K pages and the data memory is divided into banks of 128 byte, with only 80 usable bytes, resulting in an non-contiguous data memory.

It supports direct data memory access (by specifying the address as part of the instruction) and indirect data and program memory access (in an unorthodox fashion which utilize a 16 bit pointer register).

Two classes of registers exist: (8-bit class which is only one
accumulator) (16-bit class, which contains one or more 16 bit
pointer(s))

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51027 91177308-0d34-0410-b5e6-96231b3b80d8