Add alloca function (stack allocation)

alloca.S

@@ -0,0 +1,43 @@
+.intel_syntax noprefix
+
+.macro  alloca size
+sub rsp, \size
+.endm
+
+.text
+
+.globl _start
+
+_start:
+    # Could just use push, but this also works.
+    alloca 16
+    mov byte ptr [rsp+15], 10
+    mov byte ptr [rsp+14], 33
+    mov byte ptr [rsp+13], 100
+    mov byte ptr [rsp+12], 108
+    mov byte ptr [rsp+11], 114
+    mov byte ptr [rsp+10], 111
+    mov byte ptr [rsp+9], 119
+    mov byte ptr [rsp+8], 32
+    mov byte ptr [rsp+7], 44
+    mov byte ptr [rsp+6], 111
+    mov byte ptr [rsp+5], 108
+    mov byte ptr [rsp+4], 108
+    mov byte ptr [rsp+3], 101
+    mov byte ptr [rsp+2], 72
+
+# Can't move imm64 to memory directly, so have to copy to register first!
+#    mov rax, 729975031549884192
+#    mov qword ptr [rsp+8], rax
+#    mov rax, 48857072035144
+#    mov qword ptr [rsp], rax
+
+    mov rax, 1
+    mov rdi, 1
+    lea rsi, [rsp]
+    mov rdx, 15
+    syscall
+
+    mov rax, 60
+    mov rdi, 0
+    syscall