RISCV Add FPU context save (#1250)

RISCV Add FPU context save (#1250)

* port: riscv: Split the number of registers and the size of the context

* port: riscv: Create some macros for the FPU context

* port: riscv: Add a couple of macros that store fpu context

* port: riscv: Update the stack init function to include the fpu context size

* port: riscv: Add a chip_specific_extensions file that includes the F extension

* Update dictionary to include some risc-v instructions

* port: riscv: Fix a few typos

* port: riscv: Apply @aggarg's sugestions