add hello world example for RISC-V 32 and 64 bit

examples/hello-riscv/README.md

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+# Hello World in RISC-V
+
+This example shows how to assemble, link and query a simple Hello World program for RISC-V.
+
+If you want to run this example, you will need a cross-compiling toolchain like the [RISC-V GNU Compiler Toolchain](https://github.com/riscv-collab/riscv-gnu-toolchain) and [QEMU](https://www.qemu.org/).
+
+## 32-bit
+
+Assemble `hello.s` into an object file for the RISC-V 32-bit base integer instruction set (`-march rv32i`), little-endian (`-mlittle-endian`), with an ABI that follows the convention where `int`, `long` and `pointer` types are all 32-bit, with debug symbols included in the object file (`-g`):
+
+```sh
+riscv64-elf-as -march rv32i -mabi ilp32 -mlittle-endian -o hello.o hello.s -g
+```
+
+Link the object file into a RISC-V 32-bit little-endian executable (`-m elf32lriscv`), with the symbol `_start` as its entry point:
+
+```sh
+riscv64-elf-ld -e _start -m elf32lriscv -o exe --verbose hello.o
+```
+
+Execute the RISC-V ELF in QEMU:
+
+```sh
+qemu-riscv32 exe
+```
+
+Double check the disassembly:
+
+```sh
+riscv64-elf-objdump --disassemble exe
+```
+
+## 64-bit
+
+Assemble `hello.s` into an object file for the RISC-V 64-bit base integer instruction set (`-march rv64i`), little-endian (`-mlittle-endian`), with an ABI that follows the convention where `long` and `pointer` types are all 64-bit, with debug symbols included in the object file (`-g`):
+
+```sh
+riscv64-elf-as -march rv64i -mabi lp64 -mlittle-endian -o hello.o hello.s -g
+```
+
+Link the object file into a RISC-V 64-bit little-endian executable (`-m elf64lriscv`), with the symbol `_start` as its entry point:
+
+```sh
+riscv64-elf-ld -e _start -m elf64lriscv -o exe --verbose hello.o
+```
+
+Execute the RISC-V ELF in QEMU:
+
+```sh
+qemu-riscv64 exe
+```
+
+Double check the disassembly:
+
+```sh
+riscv64-elf-objdump --disassemble exe
+```

examples/hello-riscv/hello.s

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+.section .text
+.globl _start
+.equ STDOUT, 1 # File descriptor 1 is standard output (stdout)
+.equ WRITE, 64 # Linux write syscall
+.equ EXIT, 93 # Linux exit syscall
+.equ EXIT_CODE_SUCCESS, 0
+
+_start:
+    # In C, a list of parameters is passed to the kernel in a certain sequence.
+    # For the write system call, the parameters are structured as follows:
+    # ssize_t write(int fd, const void *buf, size_t count)
+    # The three parameters passed are:
+    # 1. a file descriptor (e.g. 1 for stdout)
+    # 2. a pointer to a character buffer (i.e. a string)
+    # 3. the number of characters in that string to be written. 
+    li a0, STDOUT
+    la a1, buf_begin
+    # Load a byte from memory, zero-pad it (to a 64-bit value in RV64), and store
+    # the unsigned value in the destination register a2.
+    lbu a2, buf_size
+
+    # Store the system call number in register a7.
+    li a7, WRITE
+    # Switch to RISC-V supervisor mode (the Linux kernel runs in this mode) and
+    # make a request using the value stored in a7 as the system call number.
+    ecall
+
+    li a0, EXIT_CODE_SUCCESS
+    li a7, EXIT
+    ecall
+
+# The .rodata section of an ELF binary contains constant values. The .rodata
+# section is marked as read-only, so these values cannot change at runtime.
+.section .rodata
+
+buf_begin:
+    .string "Hello World!\n"
+buf_size:
+    # Current address (the .) minus address of buf_begin = length of buffer.
+    # We store the result in a 8-bit word using the .byte directive.
+    .byte .-buf_begin