Add community triple arm64-linux and arm-linux. (microsoft#11880)

Verify the following command on both 32 bits and 64 bits OS on Raspberry pi 3.

    ./bootstrap-vcpkg.sh
    ./vcpkg install boost

src/vcpkg.cpp

@@ -235,6 +235,14 @@ int main(const int argc, const char* const* const argv)
 
     load_config(fs);
 
+#if (defined(__aarch64__) || defined(__arm__) || defined(_M_ARM) || defined(_M_ARM64)) && !defined(_WIN32)
+    if (!System::get_environment_variable("VCPKG_FORCE_SYSTEM_BINARIES").has_value())
+    {
+        Checks::exit_with_message(VCPKG_LINE_INFO,
+                                  "Environment variable VCPKG_FORCE_SYSTEM_BINARIES must be set on arm platform.");
+    }
+#endif
+
     VcpkgCmdArguments args = VcpkgCmdArguments::create_from_command_line(fs, argc, argv);
     args.imbue_from_environment();
     args.check_feature_flag_consistency();

src/vcpkg/triplet.cpp

@@ -87,7 +87,13 @@ namespace vcpkg
 #elif defined(__FreeBSD__)
                 return Triplet::from_canonical_name("x64-freebsd");
 #elif defined(__GLIBC__)
+#if defined(__aarch64__)
+                return Triplet::from_canonical_name("arm64-linux");
+#elif defined(__arm__)
+                return Triplet::from_canonical_name("arm-linux");
+#else
                 return Triplet::from_canonical_name("x64-linux");
+#endif
 #else
                 return Triplet::from_canonical_name("x64-linux-musl");
 #endif