Add strcpy function

strcpy.S

@@ -0,0 +1,48 @@
+    .intel_syntax noprefix
+
+    .globl _start
+
+    .text
+
+
+# String-copying routine.
+# Commented-out lines would also return the starting address of the destination address.
+strcpy:
+#    push rsi
+.loop:
+    movb r10b, [rdi]
+    cmpb r10b, 0
+    je .end
+    movb [rsi], r10b
+    inc rdi
+    inc rsi
+    jmp .loop
+.end:
+#    pop rax
+    ret
+
+_start:
+
+    mov rdi, offset str2
+    mov rsi, offset str1
+    call strcpy
+    # mov rsi, rax  -- would copy the string start address to rsi
+
+    mov rax, 1
+    mov rdi, 1
+    mov rsi, offset str1
+    mov rdx, 14
+    syscall
+
+    # exit
+    mov rax, 60
+    mov rdi, 0
+    syscall
+
+    .bss
+str1:
+    .skip 30
+
+    .data
+str2:
+    .string "Hello, world!\n"