Some portability improvements from trying to build with Visual Studio 2017

[mgroeber9110]

I tried getting llama.cpp to build on a machine that only had Visual Studio 2017 installed. In trying to get my build to work came across a few things that look like potential obstacles for portability, so I am proposing them as a patch, even if Visual Studio 2017 is probably not a major target in itself.

Here is a summary of the changes:

• Replaced remaining instances of restrict keyword by GGML_RESTRICT macro
• For MSVC versions that do not activate the C11 or C17 standards, use __restrict keyword instead of restrict in plain C mode (this is the only change that is really useful only for VS 2017)
• Added include <algorithm> for std::min and std::max (as per https://en.cppreference.com/w/cpp/algorithm/min)
• Added include <cctype> for std::toupper and std::tolower (as per https://en.cppreference.com/w/cpp/string/byte/tolower)
• Moved _USE_MATH_DEFINES to the top of the source file to ensure including M_PI regardless of which header it is in

For completeness, I was eventually able to build and run successfully with Visual Studio 2017, but only after commenting out the check using codecvt_utf8<char32_t> in fs_validate_filename(), as this specialization is not available in the runtime library of VS 2017. Fixing this would probably need implementing a different type of check for non-standard UTF-8 encodings, so this would only be worthwhile if support of VS 2017 is desired.