Make frontend endian-aware.

In many parts the DMD frontend assumes a little endian CPU. In some
parts there are checks for endianess but they are incomplete and the
used definition is wrong. (Test for endianess will be removed in dmd
2.062.)
In this commit I add the required #if's and also add a CMake test for
endianess because there is no single compiler definition to check for.

dmd2/module.c

@@ -544,20 +544,46 @@ bool Module::read(Loc loc)
 
 inline unsigned readwordLE(unsigned short *p)
 {
+#if IN_LLVM
+#if __LITTLE_ENDIAN__
+    return *p;
+#else
+    return (((unsigned char *)p)[1] << 8) | ((unsigned char *)p)[0];
+#endif
+#else
 #if LITTLE_ENDIAN
     return *p;
 #else
     return (((unsigned char *)p)[1] << 8) | ((unsigned char *)p)[0];
 #endif
+#endif
 }
 
 inline unsigned readwordBE(unsigned short *p)
 {
+#if IN_LLVM
+#if __BIG_ENDIAN__
+    return *p;
+#else
     return (((unsigned char *)p)[0] << 8) | ((unsigned char *)p)[1];
+#endif
+#else
+    return (((unsigned char *)p)[0] << 8) | ((unsigned char *)p)[1];
+#endif
 }
 
 inline unsigned readlongLE(unsigned *p)
 {
+#if IN_LLVM
+#if __LITTLE_ENDIAN__
+    return *p;
+#else
+    return ((unsigned char *)p)[0] |
+        (((unsigned char *)p)[1] << 8) |
+        (((unsigned char *)p)[2] << 16) |
+        (((unsigned char *)p)[3] << 24);
+#endif
+#else
 #if LITTLE_ENDIAN
     return *p;
 #else
@@ -566,14 +592,26 @@ inline unsigned readlongLE(unsigned *p)
         (((unsigned char *)p)[2] << 16) |
         (((unsigned char *)p)[3] << 24);
 #endif
+#endif
 }
 
 inline unsigned readlongBE(unsigned *p)
 {
+#if IN_LLVM
+#if __BIG_ENDIAN__
+    return *p;
+#else
     return ((unsigned char *)p)[3] |
         (((unsigned char *)p)[2] << 8) |
         (((unsigned char *)p)[1] << 16) |
         (((unsigned char *)p)[0] << 24);
+#endif
+#else
+    return ((unsigned char *)p)[3] |
+        (((unsigned char *)p)[2] << 8) |
+        (((unsigned char *)p)[1] << 16) |
+        (((unsigned char *)p)[0] << 24);
+#endif
 }
 
 #if IN_LLVM