Mention signal fixes in NEWS

[Andrew Cagney]

Here's the entire updated text:

* Signal trampoline code overhauled

Many generic problems with GDB's signal handling code have been fixed.
These include: backtraces through non-contigious stacks; recognition
of sa_sigaction signal trampolines; backtrace from a NULL pointer
call; backtrace through a signal trampoline; step into and out of
signal handlers; and single-stepping in the signal trampoline.
These fixes were tested on i386 GNU/Linux systems that include a 2.4
kernel.
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Andrew