Cygwin: cygtls: fix context alignment

Cygwin: cygtls: fix context alignment

A hang was encountered, apparently triggered by commit 63b503916d42,
changing tls_pathbufs from malloc'ed to HeapAlloc'ed memory.  After
lengthy debugging it transpired that adding the heap handle to the
tls_pathbuf struct added 8 bytes to the cygtls area, thus moving
the "context" member by 8 bytes, too, so it was suddently unaligned.

Fix this for now by changing the alignment.

Fix this once and for all, by adding code to the gentls_offsets script
to check if the alignment of the "context" member is 16 bytes.  If not,
print a matching error message, remove the just generated file, and exit
with error.

FIXME: It would be really nice to find a way to auomate the correct
alignment of the "context" member, but I don't see any way to use
alignment attributes to get what we need here.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>