Move .C to .CPP in the code (#5696)

Use g++ to compile core files to get additional C++ checks on the code.

Also move libb64 constants to PROGMEM, saving ~128 bytes of heap when used.

cores/esp8266/sqrt32.cpp

@@ -0,0 +1,60 @@
+
+#include <coredecls.h>
+#include <stdint.h>
+
+extern "C" {
+
+uint32_t sqrt32 (uint32_t n)
+{
+    // http://www.codecodex.com/wiki/Calculate_an_integer_square_root#C
+    // Another very fast algorithm donated by Tristan.Muntsinger@gmail.com
+    // (note: tested across the full 32 bits range, see comment below)
+
+    // 15 iterations (c=1<<15)
+
+    unsigned int c = 0x8000;
+    unsigned int g = 0x8000;
+
+    for(;;)
+    {
+        if (g*g > n)
+            g ^= c;
+        c >>= 1;
+        if (!c)
+            return g;
+        g |= c;
+    }
+}
+
+/*
+ * tested with:
+ *
+
+#include <stdio.h>
+#include <stdint.h>
+#include <math.h>
+
+int main (void)
+{
+    for (uint32_t i = 0; ++i; )
+    {
+        uint32_t sr = sqrt32(i);
+        uint32_t ifsr = sqrt(i);
+
+        if (ifsr != sr)
+            printf("%d: i%d f%d\n", i, sr, ifsr);
+
+        if (!(i & 0xffffff))
+        {
+            printf("%i%% (0x%08x)\r", ((i >> 16) * 100) >> 16, i);
+            fflush(stdout);
+        }
+    }
+
+    printf("\n");
+}
+
+ *
+ */
+
+};