Use __NEWLIB__ instead of __SWITCH__ and __CYGWIN__ for alloca.h-detection (#3070)

Cygwin uses newlib, so it's covered by the __NEWLIB__ check.
You can see how it defines __NEWLIB__ here: https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=newlib/configure.in#l453

imgui_draw.cpp

@@ -36,7 +36,7 @@ Index of this file:
 
 #include <stdio.h>      // vsnprintf, sscanf, printf
 #if !defined(alloca)
-#if defined(__GLIBC__) || defined(__sun) || defined(__CYGWIN__) || defined(__APPLE__) || defined(__SWITCH__)
+#if defined(__GLIBC__) || defined(__sun) || defined(__APPLE__) || defined(__NEWLIB__)
 #include <alloca.h>     // alloca (glibc uses <alloca.h>. Note that Cygwin may have _WIN32 defined, so the order matters here)
 #elif defined(_WIN32)
 #include <malloc.h>     // alloca