Cygwin: revamp TLS offsets computation

Cygwin: revamp TLS offsets computation

- convert gentls_offsets to a shell script, only running the target
  compiler and gawk.

- Simplify cygtls.h.  The new gentls_offsets script only requires two
  lines with the "public:" keyword as markers.  The comments are not
  used anymore, the output is a preprocesses file without comments.
  Align Makefile rules accordingly.

- Rather than generating perl variables and C #defines, just generate
  .ecu statements and .include the TLS offsets file right from the
  generated assembler file sigfe.s.  It's the only place we really
  need (some of) the offsets.

- Drop the target-specific name of the TLS offsets file and generate
  it on the fly in the build dir.  Fix configure and Makefile rules
  accordingly.

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