Use va_copy instead of __va_copy to fix building libomp against musl libc

[plevine]

Summary

Building libomp in Gentoo Linux with musl as libc using GCC-6.3.0 or GCC-5.4.0 results in:

/var/tmp/portage/sys-libs/libomp-9999/work/libomp-9999/runtime/src/kmp_str.cpp:171:28: error: ‘__va_copy’ was not declared in this scope
       __va_copy(_args, args); // Make copy of args.
                            ^

musl had previously provided __va_copy but has since relegated it only for scenarios in which __GNUC__ < 3, in favor of using va_copy (see http://git.musl-libc.org/cgit/musl/commit/?id=def0af189871a499efdc9bc37438d8b20eb702ab).

Seeing as libomp source code already uses one-line, double-slash style comments which implies a dependency on at least C99 standard dialect, and that va_copy is a C99 standard macro, it would seem the best course of action would be to change __va_copy to va_copy.

Fixes https://bugs.llvm.org/show_bug.cgi?id=34040

[jlpeyton]

All our sources are compiled as C++11 files and va_copy() is part of the C++11 standard. So this looks good to me.

[plevine]

I'm afraid I don't have write access so I would appreciate it if someone could commit this for me. Thanks.

openmp/trunk/runtime/src/kmp_str.cpp

/* On Linux* OS Intel(R) 64, vsnprintf() modifies args argument, so vsnprintf()
    iteration.
 
    Unfortunately, standard va_copy() macro is not available on Windows* OS.
    However, it seems vsnprintf() does not modify args argument on Windows* OS.
 */
 
 #if !KMP_OS_WINDOWS
       va_list _args;
-      __va_copy(_args, args); // Make copy of args.
+      va_copy(_args, args); // Make copy of args.
 #define args _args // Substitute args with its copy, _args.
 #endif // KMP_OS_WINDOWS
       rc = KMP_VSNPRINTF(buffer->str + buffer->used, free, format, args);
 #if !KMP_OS_WINDOWS
 #undef args // Remove substitution.
       va_end(_args);
 #endif // KMP_OS_WINDOWS
     }