Windows gdb: cygwin_set_dr => windows_set_dr, etc.

The Windows backend functions that manipulate the x86 debug registers
are called "cygwin_foo", which is outdated, because native MinGW gdb
also uses those functions, they are not Cygwin-specific.  Rename them
to "windows_foo" to avoid confusion.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I46df3b44f5272adadf960da398342a3cbdb98533
commit-id:896523e0