buildsys: order lib/ehframe-term.o after the generated headers

A parallel build fails on arm:

  CC lib/ehframe-term.o
  In file included from ./include/libc-symbols.h:49,
                   from <command-line>:
  ./libc/sysdeps/linux/arm/bits/uClibc_arch_features.h:73:11: fatal error:
      bits/uClibc_config.h: No such file or directory

Nothing orders that compile after the header generation.  "all" has pregen,
libs and startfiles as three prerequisites, and make builds the
prerequisites of a target in parallel -- "libs: pregen" does not put pregen
before the objects libs also depends on.  Every object built straight into
lib/ therefore says so itself, and the line right below this one does it for
the startfiles:

  $(CRTS) $(CTOR_TARGETS) $(CRTS_COMPAT) $(CRTRELOC): | headers

0a688ba5b ("buildsys: terminate .eh_frame in shared objects") added a second
such object without that edge.

arm is the only architecture that notices: its bits/uClibc_arch_features.h
includes the generated bits/uClibc_config.h to check __CONFIG_ARM_EABI__
against the compiler's __ARM_EABI__.  Elsewhere the file compiles without it
and the missing edge stays invisible.  It also stays invisible wherever an
earlier install left a bits/uClibc_config.h in the include path; a toolchain
sysroot that is being rebuilt has none, which is why this appears when the
toolchain and the library are built together.

Order-only, like the startfiles line above: the header is regenerated on
every configuration change, and a normal prerequisite would recompile the
object each time for nothing.

Reported-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Ramin Moussavi <lordrasmus@gmail.com>

Makerules

@@ -433,7 +433,7 @@ endif
 # we link with -nostdlib, so gcc's crtendS.o never provides it
 EHFRAME_TERM := $(top_builddir)lib/ehframe-term.o
 CFLAGS-ehframe-term.c := -fno-unwind-tables -fno-asynchronous-unwind-tables
-$(EHFRAME_TERM): $(top_builddir)lib/%.o : $(top_srcdir)libc/sysdeps/linux/common/%.c | $(top_builddir)lib
+$(EHFRAME_TERM): $(top_builddir)lib/%.o : $(top_srcdir)libc/sysdeps/linux/common/%.c | headers $(top_builddir)lib
 	$(compile.c)
 # both the soname targets and the recipe-bearing unversioned targets
 # (the ldso rule runs link.so on $(ldso:.$(ABI_VERSION)=))