Enable ASM_MASM instead of ASM on MSVC.

[Nekto89]

Without this change rdrand.asm, x64dll.asm and x64masm.asm do not participate in build.

[abdes]

I think there is a bigger issue with handling the special files such as asm files in the build. I'm not comfortable yet with the change you propose in your pull request as you just solved the issue for MSVC. We need a bit more time to review all the changes necessary to properly handle asm compilation.

EDIT 1: Further review of the CMakeLists.txt =>

• we enable language ASM_MASM but set the source property to ASM. That's an issue.
• asm files only added to sources for MSVC -> OK

Changes proposed by @Nekto89 fix the build (which was failing) on appveyor.

==> Suggest to accept his pull request (@noloader )