[dynarmic] fix sign extension and optimize vaddr lookups for 64bit on x64 systems (#4446)

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------
Same as #4415 + #4444 just ported to A64 because I amazingly forgot

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4446
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/dynarmic/src/dynarmic/backend/x64/emit_x64_memory.h

@@ -161,8 +161,12 @@ template<>
         code.jnz(abort, code.T_NEAR);
     }
 
-    code.shl(tmp, int(ctx.conf.page_table_log2_stride));
-    code.mov(page, qword[r14 + tmp]);
+    if (ctx.conf.page_table_log2_stride > 3) {
+        code.shl(tmp, int(ctx.conf.page_table_log2_stride));
+        code.mov(page, qword[r14 + tmp.cvt64()]);
+    } else {
+        code.mov(page, qword[r14 + tmp.cvt64() * int(1 << ctx.conf.page_table_log2_stride)]);
+    }
 
     // check for marked bit, use as unmapped if marked
     if (ctx.conf.page_table_marked_bit) {
@@ -178,9 +182,10 @@ template<>
         code.mov(tmp, ctx.conf.page_table_pointer_mask);
         code.and_(page, tmp);
     }
+    // check for sign bit, apply sign extension as needed
     if (ctx.conf.page_table_sign_extension) {
-        code.shl(page, *ctx.conf.page_table_sign_extension);
-        code.sar(page, *ctx.conf.page_table_sign_extension);
+        code.shl(page, 63 - int(*ctx.conf.page_table_sign_extension));
+        code.sar(page, 63 - int(*ctx.conf.page_table_sign_extension));
     }
 
     code.jz(abort, code.T_NEAR);