gdb: put user-supplied CFLAGS at the end

GDB currently doesn't build cleanly with clang (a -Wdeprecated-copy-dtor
error).  I configured my clang-based GDB build with
CXXFLAGS="-Wno-error=deprecated-copy-dtor", so I can use it despite that
problem.  However, I found that it had no effect.  This is because my
-Wno-error=Wdeprecated-copy-dtor switch is followed by -Werror in the
command line, which switches back all warnings to be errors.

If we want the user-supplied C(XX)FLAGS to be able to override flags
added by our configure script, the user-supplied C(XX)FLAGS should
appear after the configure-supplied flags.

This patch moves the user-supplied CXXFLAGS at the very end of the
compilation command line, which fixes the problem described above.  This
means moving it out of INTERNAL_CFLAGS and inlining it in the users of
INTERNAL_CFLAGS.

I observed the problem when building GDB, but the same problem could
happen with GDBserver, so the change is done there too.

In GDBserver, INTERNAL_CFLAGS is passed when linking

gdb/ChangeLog:

	* Makefile.in (COMPILE): Add CXXFLAGS.
	(INTERNAL_CFLAGS_BASE): Remove CXXFLAGS.
	(check-headers): Add CXXFLAGS.

gdbserver/ChangeLog:

	* Makefile.in (COMPILE): Add CXXFLAGS.
	(INTERNAL_CFLAGS_BASE): Remove CXXFLAGS.
	(gdbserver$(EXEEXT)): Add CXXFLAGS.
	(gdbreplay$(EXEEXT)): Add CXXFLAGS.
	($(IPA_LIB)): Add CXXFLAGS.
	(IPAGENT_COMPILE): Add CXXFLAGS.

Change-Id: I00e054506695e0e9536095c6d14827e48abd8f69