make: create native tcc from separate objects

This was already possible using
    make NOTALLINONE=1
and is now the default.

To build as previously from one big source, use
    make ONE_SOURCE=1

Cross compilers are still build from one source because using
separate objects requires separate build directories one per
platform which currently is not (yet) supported by the makefile.

We could probably use gnu-makeish target variables like
    $(I386_CROSS): OUTDIR=build/i386
    $(X64_CROSS): OUTDIR=build/x86-64
and so on ...

Also NEED_FLOAT_TYPES for arm-gen is removed.  It was about
variables that are referenced from outside (libtcc, tccgen).
We could declare them in tcc.h (as with reg_classes) or have
them twice in arm-gen.c.  I chose option 2.

tccpe.c

@@ -376,7 +376,7 @@ static int pe_find_import(TCCState * s1, ElfW(Sym) *sym)
 {
     char buffer[200];
     const char *s, *p;
-    int sym_index, n = 0;
+    int sym_index = 0, n = 0;
 
     do {
         s = pe_export_name(s1, sym);