#3174 crashes in tclWinFCmd.c

[Kevin B KENNY]

Pat Thoyts reports, and I confirm, that configuring
a mingw build [gcc.exe (GCC) 3.4.2 (mingw-special)]
with --enable-symbols=all --enable-threads
gives rise to panics in the DoCopyFile and DoRenameFile
procedures of tclWinFCmd.c.

The problem is that the blocks of asm code do not
conserve the stack pointer and contain transfers of
control from one to another; both of these are
illegal according to the gcc manual.

There is similar code in tclWinChan.c that appears
either not to be executed or else avoids the
problem by the same dumb luck that protected
tclWinFCmd.c on earlier mingw releases.

Identical code is present in both files in 8.5.

[Kevin B KENNY]

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Fixes to the assembly code applied to both the HEAD and
core-8-4-branch.