Direct compile 16bit C code instead of including via .S files.

Direct compile 16bit C code instead of including via .S files.

Create a ".code16gcc" directive in src/code16gcc.s and use
-Wa,src/code16gcc.s to tell gcc to compile directly to 16bit code.
This eliminates the need to compile the C code to assembler and
include in romlayout.S and vgaentry.S.  This also allows those two
assembler files to be compiled with debugging (-g) enabled.

Also, includes some Makefile cleanups.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>