win, build: fix building on 32-bit machines

Fixes: #21402

PR-URL: #21437
Fixes: #21402
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

vcbuild.bat

@@ -203,6 +203,8 @@ if _%PROCESSOR_ARCHITEW6432%_==_AMD64_ set msvs_host_arch=amd64
 set vcvarsall_arg=%msvs_host_arch%_%target_arch%
 @rem unless both host and target are x64
 if %target_arch%==x64 if %msvs_host_arch%==amd64 set vcvarsall_arg=amd64
+@rem also if both are x86
+if %target_arch%==x86 if %msvs_host_arch%==x86 set vcvarsall_arg=x86
 
 @rem Look for Visual Studio 2017
 :vs-set-2017