libc/newlib: build in the 'start files' pass

A few facts:
- building the C library requires a proper core compiler
- core compiler is issued from one of the core passes
- the C library is required to build libstdc++
- newlib is only built for baremetal
- in bare metal, the final compiler is issued from one of the core passes

So we need to build the C library between core pass 1 and core pass 2.
The only place is eithe libc_headers() or libc_start_files(). The most
pertinent seems to be libc_start_files().

So we build newlib from libc_start_files(), and leave libc() empty.

scripts/build/libc/newlib.sh

@@ -59,10 +59,6 @@ do_libc_headers() {
 }
 
 do_libc_start_files() {
-    :
-}
-
-do_libc() {
     CT_DoStep INFO "Installing C library"
 
     mkdir -p "${CT_BUILD_DIR}/build-libc"
@@ -84,18 +80,20 @@ do_libc() {
         --host=${CT_BUILD}                              \
         --target=${CT_TARGET}                           \
         --prefix=${CT_PREFIX_DIR}
-    
+
     CT_DoLog EXTRA "Building C library"
-
     CT_DoExecLog ALL make ${PARALLELMFLAGS}
-    
-    CT_DoLog EXTRA "Installing C library"
 
+    CT_DoLog EXTRA "Installing C library"
     CT_DoExecLog ALL make install install_root="${CT_SYSROOT_DIR}"
 
     CT_EndStep
 }
 
+do_libc() {
+    :
+}
+
 do_libc_finish() {
     CT_DoStep INFO "Finishing C library"