merge GCC 10.4 into gcc.old.

merge GCC 8.4 into gcc.old.

merge gcc 7.4 into gcc.old part 3 (last?)

delete GCC 6.5. every platform has been on GCC 7 for a while now.

merge GCC 6.5.0 into gcc.old.

merge in missing changes from gcc to gcc.old (both GCC 6.4.):

--
redo mknative-gcc for all ports. main changes include:
- "#define HAVE_CC_TLS 1" for most/all ports, thanks maya@
- "#define _GLIBCX

merge in missing changes from gcc to gcc.old (both GCC 6.4.):

--
redo mknative-gcc for all ports. main changes include:
- "#define HAVE_CC_TLS 1" for most/all ports, thanks maya@
- "#define _GLIBCXX_HAVE_LDEXPL 1" and "#define _GLIBCXX_HAVE_TGMATH_H 1"
for many ports
- arm64 and amd64 had a broken c++config.h that disabled many things
- configargs.h has more normalisation
- ppc64 has a few things fixed, must have missed several mknative rounds
--
regen to pull out INTERNAL_CFLAGS.
--
pull -DHAVE_CC_TLS out of $(INTERNAL_CFLAGS) if it is there, and add
it to CPPFLAGS.

this fixes PR#53567 for me.

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import GCC 6.4 srcs into gcc.old in preparation for GCC 7.x.

these are netbsd GCC srcs from 20180801.