This implements #7158 by conditionally choosing a UA string by
`#[cfg()]`-checking for `target_os = linux` and whether `target_arch` is
`x86_64` or not. Matching the behavior of Firefox, either "X11; Linux
x86_64" or "X11; Linux i686" is included.
`target_os = windows` is also checked; again as in Firefox "Windows NT
6.1; Win64; x64" or just "Windows NT 6.1" is included. The UA string
pretends to be non-WoW64 Windows 7, since there's only so much we can
detect at build time.
The existing desktop UA string that lists OS X is chosen if `target_os`
is neither `linux` nor `windows`.