Define G_MSVC_SYMBOL_PREFIX correctly for ARM
requested to merge stoegglw/glib:2019-10-28_Define_G_MSVC_SYMBOL_PREFIX_correctly_for_ARM into master
Symbols on x86 are prefixed with an underscore but symbols on x64/ARM/ARM64 are not.
Symbols on x86 are prefixed with an underscore but symbols on x64/ARM/ARM64 are not.