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.

LibC/fd_set.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#define FD_SETSIZE 64
+#define FD_ZERO(set) memset((set), 0, sizeof(fd_set));
+#define FD_CLR(fd, set) ((set)->bits[(fd / 8)] &= ~(1 << (fd) % 8))
+#define FD_SET(fd, set) ((set)->bits[(fd / 8)] |= (1 << (fd) % 8))
+#define FD_ISSET(fd, set) ((set)->bits[(fd / 8)] & (1 << (fd) % 8))
+
+struct __fd_set {
+    unsigned char bits[FD_SETSIZE / 8];
+};
+
+typedef struct __fd_set fd_set;