arch: add riscv

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diff --git a/src/arch/riscv.md b/src/arch/riscv.md
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+# riscv
+keywords: rv32, rv64
+
+- ISA type: `RISC`
+- Endianness: `little`, `big`
+
+## Registers
+- riscv32 => `XLEN=32`
+- riscv64 => `XLEN=64`
+
+### General purpose registers
+```markdown
+[XLEN-1:0]     abi name     desc
+---------------------------------------------
+x0             zero         zero register
+x1             ra           return addr
+x2             sp           stack ptr
+x3             gp           global ptr
+x4             tp           thread ptr
+x5-x7          t0-t2        temp regs
+x8-x9          s0-s1        saved regs
+x10-x17        a0-a7        arg regs
+x18-x27        s2-s11       saved regs
+x28-x31        t3-t6        temp regs
+```
+
+## ASM skeleton
+Small assembler skeleton, ready to use with following properties:
+- use raw Linux syscalls (`man 2 syscall` for ABI)
+- no `C runtime (crt)`
+- gnu assembler [`gas`][gas_doc]
+```mipsasm
+// file: greet.S
+
+#include <asm/unistd.h>     // syscall NRs
+
+    .section .text, "ax", @progbits
+    .balign 4               // align code on 4byte boundary
+    .global _start
+_start:
+    li a0, 2                // fd
+    la a1, greeting         // buf
+    ld a2, (greeting_len)   // &len
+    li a7, __NR_write       // write(2) syscall
+    ecall
+
+    li a0, 42               // exit code
+    li a7, __NR_exit        // exit(2) syscall
+    ecall
+
+    .balign 8               // align data on 8byte boundary
+    .section .rodata, "a", @progbits
+greeting:
+    .asciz "Hi ASM-World!\n"
+greeting_len:
+    .int .-greeting
+```
+> man gcc: `file.S` assembler code that must be preprocessed.
+
+To cross-compile and run:
+```bash
+> riscv64-linux-gnu-gcc -o greet greet.S -nostartfiles -nostdlib                \
+    -Wl,--dynamic-linker=/usr/riscv64-linux-gnu/lib/ld-linux-riscv64-lp64d.so.1 \
+  && qemu-riscv64 ./greet
+Hi ASM-World!
+```
+> Cross-compiling on `Ubuntu 20.04 (x86_64)`, paths might differ on other
+> distributions. Explicitly specifying the dynamic linker should not be
+> required when compiling natively on riscv.
+>
+> Select dynamic linker according to abi used during compile & link.
+
+## References
+- [GNU Assembler][gas_doc]
+- [GNU Assembler Directives][gas_directives]
+
+[gas_doc]: https://sourceware.org/binutils/docs/as
+[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops