Update RTE to support easy emulation of instructions

[stnolting]

This PR updates the NEORV32 Runtime Environment (RTE) to support easier emulation of unsupported CPU / SoC features like unimplemented instructions and even unaligned memory accesses.

The RTE now performs a full context backup of the interrupted application by storing all x integer registers to the stack. The base address of this stack frame is copied to the mscratch CSR. Dedicated "context access" function use this CSR value to provide easy access to the application context allowing reading and altering of registers:

// Prototypes
uint32_t neorv32_rte_context_get(int x); // read register x
void     neorv32_rte_context_put(int x, uint32_t data); write data to register x

// Examples
uint32_t tmp = neorv32_rte_context_get(9); // read register 'x9'
neorv32_rte_context_put(28, tmp); // write 'tmp' to register 'x28'

🚀 A demo program is added to sw/example/demo_emulate_unaligned to showcase how to emulate unaligned memory accesses.