merge GCC 10.5.0 in gcc.old

merge GCC 10.4 into gcc.old.

merge GCC 9.3 into gcc.old.

merge GCC 8.4 into gcc.old.

regen mknative files for GCC 7.5.

Switch the iconv(3) prototype to the POSIX conformant variation

Remove const from the 2nd argument.

const char ** and char ** are incompatible types and it was a cost to keep
the technically incomp

Switch the iconv(3) prototype to the POSIX conformant variation

Remove const from the 2nd argument.

const char ** and char ** are incompatible types and it was a cost to keep
the technically incompatible form for a more purist variation. NetBSD was
almost the last alive OS to still keep the const argument (known leftovers:
Minix and Illumos).

Keep the const form for the internal purposes inside citrus and rump.

Address the build breakage fallout in the same change.

There are no ABI changes.

Change accepted by core@.

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import GCC 7.4 as gcc.old