Building newlib with MSVC would take a large porting effort. You are better off porting your code to the libc provided by MSVC. They should be mostly compatible. Simply remove newlib from you build system, MSVC will automatically link you code against it's own libc.
If you can built your code under MSVC you've probably already ported it to MSVC's libc anyway. Unless you are explicitly including headers from newlib. For example, if you include stdio.h
, by default it will pickup MSVC's version unless you override this behavior to get it to use newlib's version.