Support both MingGW multilib versions

Fix #10941, close #10962

BUILD.md

@@ -55,8 +55,11 @@ MinGW-w64 can be downloaded [here](https://sourceforge.net/projects/mingw-w64/fi
 3. Run `mingw32-make`.
 4. The 32-bit or 64-bit `notepad++.exe` will be generated either in `bin.i686` or in `bin.x86_64` directory respectively, depending on the target CPU of the compiler — look for the full path to the resulting binary at the end of the build process.
 
+### Some additional information:
+
 * The directory containing `notepad++.exe` will also contain everything needed for Notepad++ to start.
 * To have a debug build just add `DEBUG=1` to the `mingw32-make` invocation above. The output directory then will be suffixed with `-debug`.
 * To see commands being executed add `VERBOSE=1` to the same command.
-* When switching between compilers or between release/debug modes, `mingw32-make clean` must be executed first.
-* If you are building the project outside of the `PowerEditor/gcc` directory, for example when using `-f` option, then the entire project path must not contain any spaces. Additionally, the path to `makefile` of this project should be listed as first.
+* When switching between compilers or between release/debug modes cleaning must be done first to delete previous temporary files. This can be done by invoking `mingw32-make clean` (the directory with `notepad++.exe` won't be affected) or `mingw32-make fullclean` (everything related to the build will be deleted, including the directory with `notepad++.exe`).
+* When a project is built outside of the `PowerEditor/gcc` directory, for example when using `-f` option, then the entire project path must not contain any spaces. Additionally, the path to `makefile` of this project should be listed as first.
+* When a project is built through MinGW-w64 with multilib support, a specific target can be forced by passing `TARGET_CPU` variable with `x86_64` or `i686` as value.

PowerEditor/gcc/makefile

@@ -50,7 +50,7 @@ SUBMAKEFLAGS := -O --no-print-directory
 ifeq "$(filter-out 0,$(DEBUG))" ""
 BUILD_TYPE := release
 BUILD_SUFFIX :=
-CXXFLAGS += -O2 -Os
+CXXFLAGS += -Os
 CPP_DEFINE += NDEBUG
 LDFLAGS += -s
 else
@@ -81,12 +81,17 @@ ifeq "$(TARGET_CPU)" "i686"
 ifeq "$(wildcard $(WIN_DIR)/system32/SensApi.dll)" ""
 $(error $(TARGET_CPU) build requires "%windir%/system32/SensApi.dll" to be present)
 endif
-# detect explicit definition of TARGET_CPU via command line to force a 32-bit build
+# detect explicit definition of TARGET_CPU via command line to force a 32-bit build for MinGW-w64 with support multilib
 ifeq "$(origin TARGET_CPU)" "command line"
 export CXX += -m32
 export LD += -m32
 export RC += -Fpe-i386
 endif
+# detect explicit definition of TARGET_CPU via command line to force a 64-bit build for MinGW-w64 with support multilib
+else ifneq "$(and $(findstring x86_64,$(TARGET_CPU)),$(findstring command line,$(origin TARGET_CPU)))" ""
+export CXX += -m64
+export LD += -m64
+export RC += -Fpe-x86-64
 endif
 
 # define target and build directories and update dependent variables