build.bat: Fix bug introduced by 520944bf5fdf

Commit 520944bf5fdf accidently reversed the if/else-check for InnoSetup
and 7z. This caused release packaging to break on both 32bit and 64bit windows.

This fixe works around this by introducing a new variable %os_type% which
stores Win32 or Win64, and uses this to determine the path of InnoSetup
and 7z.

The variable %x64_type%, which decided on wether to use the amd64 native compiler
or the x86_amd64 32bit compiler for a 64 bit build was removed completely, because
visual studio always uses x64_amd64. Hence hardcode this value when setting ARCH=.