SAMV7 Ethernet: Fix a compiler error introduced with commit 7467329a

Add debug assertion in libdtoa to catch attempts to use floating point output formats from within an interrupt handler.  That will cause assertions or crashes downstream because __dtoa will attempt to allocate memory.

Add configuration to prevent selection of Windows native toolchains when using Ubuntu under Windows 10

Back out most of 34be3e7c and update README again.  Windows native tools cannot be used with Ubuntu under Windows 10 now.  For Cygwin, that support depends on the 'cygpath -w' tool to convert POSIX paths to Windows paths.  There is no corresponding tool for Ubuntu under Windows 10.