clarify ARM64EC support (#169)

BUILD.md

@@ -80,9 +80,8 @@ For Windows 7 or later:
 
 Notes for Windows on ARM:
 - Red Panda C++ can be built for ARM64 ABI only on Windows 11 ARM64, while it is supposed (but not tested) to run on Windows 10 ARM64.
-- The ARM64EC (“emulation compatible”) ABI, which highlights interoperability with x64, is currently not supported.
-  - ARM64EC does not bring significant benefit, since Red Panda C++ can be built to ARM64 classic ABI.
-  - However, ARM64EC will allow users to use their favorite input methods, fancy Qt styles.
+  - ARM64EC (“emulation compatible”) host is not supported, i.e., Red Panda C++ cannot be built with ARM64EC toolchain.
+  - ARM64EC target is (theoretically) supported, i.e. Red Panda C++ will build ARM64EC binaries if upstream toolchain supports ARM64EC.
 - With the [ARM32 deprecation in Windows 11 Insider Preview Build 25905](https://blogs.windows.com/windows-insider/2023/07/12/announcing-windows-11-insider-preview-build-25905/), ARM32 support will never be added.
 
 For legacy Windows (NT 5.1 – 6.0):

BUILD_cn.md

@@ -80,9 +80,8 @@
 
 关于 ARM 上的 Windows 的注记：
 - 小熊猫 C++ 只能在 Windows 11 ARM64 上构建 ARM64 版，成品应该能在 Windows 10 ARM64 上运行（但没有测试过）。
-- 暂不支持以 x64 互操作性著称的 ARM64EC（“仿真兼容”）ABI。
-  - 既然小熊猫 C++ 已经可以构建到 ARM64 经典 ABI，ARM64EC 就不能带来明显的好处。
-  - 但是 ARM64EC 可以支持用户习惯的输入法和喜欢的 Qt 样式。
+  - 不支持 ARM64EC（“仿真兼容”）主机，即不能用 ARM64EC 工具链构建小熊猫 C++。
+  - （理论上）支持 ARM64EC 目标，也就是说，如果上游工具链支持 ARM64EC，那么小熊猫 C++ 可以构建 ARM64EC 程序和库。
 - 随着 [Windows 11 Insider Preview Build 25905 弃用 ARM32](https://blogs.windows.com/windows-insider/2023/07/12/announcing-windows-11-insider-preview-build-25905/)，小熊猫 C++ 今后也不会添加 ARM32 支持了。
 
 适用于旧版 Windows（NT 5.1 – 6.0）：

RedPandaIDE/settings.cpp

@@ -3624,7 +3624,7 @@ void Settings::Environment::doLoad()
     }
 
 #ifdef Q_OS_WINDOWS
-# if defined (__aarch64__) || defined(_M_ARM64) || defined (_M_ARM64EC)
+# if defined (__aarch64__) || defined(_M_ARM64)
     // the only native MinGW toolchain (LLVM-MinGW) does not have local codepage support
     // prefer UTF-8 compatible OpenConsole.exe
     mUseCustomTerminal = boolValue("use_custom_terminal", true);