Add Linux RISC-V 32-bit/64-bit prepends

[bcoles]

Verification

Compare the before and after for both riscv32le and riscv64le. Note the two additional syscalls.

Before:

$ ./msfvenom -f elf -p linux/riscv64le/reboot > reboot.elf
[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, selecting arch: riscv64le from the payload
No encoder specified, outputting raw payload
Payload size: 40 bytes
Final size of elf file: 160 bytes

$ /home/user/qemu/build/qemu-riscv64 -strace ./reboot.elf 
7000 reboot(4276215469,672274793,19088743,0,0,0) = -1 errno=1 (Operation not permitted)
--- SIGILL {si_signo=SIGILL, si_code=1, si_addr=0x00000000004000a0} ---
Illegal instruction (core dumped)

After:

$ ./msfvenom -f elf -p linux/riscv64le/reboot PrependSetresuid=true AppendExit=true > reboot.elf
[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, selecting arch: riscv64le from the payload
No encoder specified, outputting raw payload
Payload size: 72 bytes
Final size of elf file: 192 bytes

$ /home/user/qemu/build/qemu-riscv64 -strace ./reboot.elf 
7133 setresuid(0,0,0) = -1 errno=1 (Operation not permitted)
7133 reboot(4276215469,672274793,19088743,0,0,0) = -1 errno=1 (Operation not permitted)
7133 exit(0)