Slight change to riscv crt0 assembly 
Slight change to riscv crt0 assembly 

We need to save app_start in a0 (first argument) before overwriting s0 in crt0. This allows app_start to be passed to _c_start instead of the frame pointer (which is the expected behavior).
tock/libtock-c,tock/libtock-c,tock/libtock-c