winebuild: Conditionally use different code for restoring the context structure.

[Sebastian Lackner]

Fixes a regression introduced by 44fbc018eda12bdee5c2c1e2e40dbdc6a81b27fd.
See for example: https://bugs.winehq.org/show_bug.cgi?id=26344#c9

The previous code was not completely correct for the situation when switching
back to a 16-bit stack. If ESP contains garbage in the high word, then accessing
the memory on the target stack fails. We could theoretically use only the lower
16-bits for accessing the memory, but there is no good way to determine if a
segment if 16- or 32-bit. To ensure that the original issue stays fixed this
patch adds a check, to decide at runtime if we can safely copy values on the target
stack or not. If not then we use again the "old" method of restoring the thread
context.

---
 tools/winebuild/relay.c | 57 +++++++++++++++++++++++++++++++++++--------------
 1 file changed, 41 insertions(+), 16 deletions(-)
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