fb.bas: Remove FB_TARGETOPT_UNDERSCORE and add env.underscoreprefix instead

The new global variable is not tied to a certain target OS like the old
FB_TARGETOPT_UNDERSCORE flag, which is good, because whether we need the
leading underscores or not, does not only depend on the OS. (see next
commit)
Don't add leading underscores to ASM symbols on Win64 and Cygwin 64bit