unittests/ASM: Implements a unit test for FEX-Emu#3478

This unit test recreates the error condition that FEX-Emu#3478 causes.
With a string operation that is a backwards copy then the optimization
will read past the end of the page and result in a crash.

Seemingly only happens with backwards string operations, but test
forward and backward in this test.

unittests/ASM/FEX_bugs/repeat_stringops_crash.asm

@@ -0,0 +1,65 @@
+%ifdef CONFIG
+{
+  "MemoryRegions": {
+    "0xf0000000": "4096",
+    "0xf1000000": "4096"
+  }
+}
+%endif
+
+; FEX-Emu had a bug where a backwards repeating string operation would read past the end of a mapped page.
+; This was encountered in https://github.com/FEX-Emu/FEX/pull/3478.
+; To ensure we don't read past a page with `rep stos` and `rep movs`, map two disparate pages and copy the entire page.
+; If FEX tries reading past the ends of either then it will fault.
+%macro do_rep_op 2
+  jmp %%1
+  %%1:
+
+  cld
+  mov rax, r13
+  mov rdi, r14
+  mov rsi, r15
+  mov rcx, (4096 / %2)
+  rep %1
+%endmacro
+
+%macro do_backward_rep_op 2
+  jmp %%1
+  %%1:
+
+  std
+  mov rax, r13
+  mov rdi, r14
+  mov rsi, r15
+  add rdi, (4096 - %2)
+  add rsi, (4096 - %2)
+  mov rcx, (4096 / %2)
+  rep %1
+%endmacro
+
+mov r15, 0xf000_0000
+mov r14, 0xf100_0000
+mov r13, 0x41424344454647
+
+do_rep_op stosb, 1
+do_rep_op stosw, 2
+do_rep_op stosd, 4
+do_rep_op stosq, 8
+
+do_backward_rep_op stosb, 1
+do_backward_rep_op stosw, 2
+do_backward_rep_op stosd, 4
+do_backward_rep_op stosq, 8
+
+do_rep_op movsb, 1
+do_rep_op movsw, 2
+do_rep_op movsd, 4
+do_rep_op movsq, 8
+
+do_backward_rep_op movsb, 1
+do_backward_rep_op movsw, 2
+do_backward_rep_op movsd, 4
+do_backward_rep_op movsq, 8
+
+hlt
+