llvm-ml with "-m64" command line flag by default for 64 bit package

[marakew]

it makes sense to enable this flag by default
for packages built for a 64-bit platform

the compiler and linker use 64-bit generation by default
but the assembler cannot be used without this additional flag
this causes inconvenience when using cmake
like boost context
where cmake determines that the x64_86 architecture
but when starting to build llvm-ml.exe as for 32-bit mode

or for 64-bit platforms make an ml64 build
where this flag will be enabled by default