Use __func__ instead of __FUNCTION__ where applicable

This replaces usages of the non-standard __FUNCTION__ macro with the standard
mandated __func__ identifier.

__FUNCTION__ is a preprocessor definition that is provided as an
extension by compilers. This was the only convenient option to rely on
pre-C++11. However, C++11 and greater mandate the predefined identifier
__func__, which lets us accomplish the same thing.

The difference between the two, however, is that __func__ isn't a
preprocessor macro, it's an actual identifier that exists at function
scope. The C++17 draft standard (N4659) at section [dcl.fct.def.general]
paragraph 8 states:

"
The function-local predefined variable __func__ is defined as if a
definition of the form

static const char __func__[] = "function-name ";

had been provided, where function-name is an implementation-defined
string. It is unspecified whether such
a variable has an address distinct from that of any other object in the
program.
"

Thankfully, we don't do any macro or string concatenation with __FUNCTION__
that can't be modified to use __func__.

Source/Core/VideoBackends/D3D/D3DBase.h

@@ -35,7 +35,7 @@ namespace DX11
 #define CHECK(cond, Message, ...)                                                                  \
   if (!(cond))                                                                                     \
   {                                                                                                \
-    PanicAlert(__FUNCTION__ " failed in %s at line %d: " Message, __FILE__, __LINE__,              \
+    PanicAlert("%s failed in %s at line %d: " Message, __func__, __FILE__, __LINE__,               \
                __VA_ARGS__);                                                                       \
   }