Add compilation support with Newlib nano for embedded targets

Remove the dependency on std::locale to reduce executable sizes

For embedded target the usage of locale bring a huge amount of code to the executable, just to get hold of the thousands separator. Using a static defined thousands separator makes libfmt embedded target friendly.

include/fmt/posix.h

@@ -266,7 +266,8 @@ class file {
 long getpagesize();
 
 #if (defined(LC_NUMERIC_MASK) || defined(_MSC_VER)) && \
-    !defined(__ANDROID__) && !defined(__CYGWIN__) && !defined(__OpenBSD__)
+    !defined(__ANDROID__) && !defined(__CYGWIN__) && !defined(__OpenBSD__) && \
+    !defined(__NEWLIB_H__)
 # define FMT_LOCALE
 #endif