More compat work. Rename libraries from LibFoo.a => libfoo.a

This makes it more straightforward to build a cross-compiler toolchain.
Also move math stuff from LibC to LibM.

LibM/math.cpp

@@ -0,0 +1,23 @@
+#include <LibC/assert.h>
+#include <LibM/math.h>
+
+extern "C" {
+
+double cos(double)
+{
+    assert(false);
+}
+
+double sin(double)
+{
+    assert(false);
+}
+
+double pow(double x, double y)
+{
+    (void)x;
+    (void)y;
+    assert(false);
+}
+
+}