Fix CPack output filename on Windows

[seisman]

Description of proposed changes

CPACK_SYSTEM_NAME is empty here, resulting in gmt-6.0.0rc2-.exe on Windows. This PR fixes the issue by checking the 64 bits or 32 bits of the target architecture.

[seisman]

@joa-quim Can you check if this PR can fix the installer name problem?

You don't need to generate the installers, just run the cmake configuration, and you should see something like:

-- Performing Test HAVE_TRADITIONAL_CPP
-- Performing Test HAVE_TRADITIONAL_CPP - Failed
-- Using CFLAGS = '-Wextra -Wall -Wdeclaration-after-statement -O3 -DNDEBUG'
###gmt-6.1.0rc2-win64###
*
*  GMT Version:               : 6.1.0rc2
*
*  Options:

[joa-quim]

Yes, it prints the right name ###gmt-6.1.0rc2-win64###
Funny, I have a variable (BITAGE) in my ConfigUser.cmake that does exactly that bits detection test.