Suggested addition to documentation

[Brad Lucier]

I couldn't find the best place in the documentation to put this note.

This is true for 3.0; I believe there have been three questions
about this performance issue since last summer posted to the
gcc mail list.

Note: If you write a program using computed gotos, a GCC extension,
and you compile the program with optimization level @samp{-O2} or higher,
you may get better runtime performance if you disable the global common
subexpression elmination (gcse) pass by adding the @samp{-fno-gcse} option'
to the command line.

Brad Lucier