Clean up Windows makefiles

Clean up Windows makefiles

Get rid of the CPU and CPUTYPE variables in win64.mak. Instead, assume that
the TARGET_CPU variable has been set correctly in the environment.
TARGET_CPU is set by "setenv" script that comes with the Microsoft Visual
Studio C compiler package. It never worked to pass a "CPU=x86" and using a
x64 compiler, for example, so seems better to determine the correct target
from the environment, than fail with a cryptic error message if the CPU
argument and the environment don't match. To build 32-bit binaries with
win64.mak, simply do "setenv /x86" before running "nmake -f win64.mak".
Similarly, to build 64-bit binaries, use "setenv /x64".

It should now be possible to build both 32-bit and 64-bit binaries with
win64.mak. Ideally, we could now remove win32.mak altogether, but there are
small differences between win64.mak and win32.mak in the build options that
they use. I don't understand the differences enough to dare to remove
win32.mak yet.

The CPU=AMD64 alias for x64 is no longer supported (as the whole parameter
is now gone).

Also do whitespace & other cosmetic fixes to win32.mak and win64, to make
them more similar. This helps when comparing the two with "diff".

Remove remains of the long-gone "MULTIBYTE" setting (it was misspelled in
win64.mak, anyway)