[Build] 32-bit: Actually create `bin/Pipeline` during the 32-bit part -.-

Lol, this led the bmp2arr compilation to fail on a clean checkout.
Lesson learned: Always test on a clean clone, not in your separate test
repo that you just `git pull` up to the final commit in a push, but
with all the build artifacts still in there…

Also, yes, `mkdir` only supports backslashes.

Part of P0001, funded by GhostPhanom.

Makefile.mak

@@ -48,7 +48,6 @@ bin\piloadc.obj: libs\piloadc\piloadc.asm
 	tlink /t /3 $**
 
 bin\Pipeline\grzview.com: Pipeline\grzview.cpp th01\formats\grz.cpp
-	mkdir bin\Pipeline
 	$(CC) $(CFLAGS) -Z -mt -lt -nbin\Pipeline\ $** masters.lib
 
 bin\th01\zunsoft.com: th01\zunsoft.c

build32b.bat

@@ -20,7 +20,11 @@ REM ----------------------------------------------
 
 set CXX32FLAGS=%CXX32FLAGS% -O2 -v- -x-
 
+: Neither BCC32 nor TASM32 automatically create nonexisting output
+: directories...
+mkdir bin\Pipeline 2>NUL
 for /L %%i in (1,1,5) do mkdir bin\th0%%i 2>NUL
+
 del /S *.obj
 
 set BMP2ARR=bin\Pipeline\bmp2arr.exe