gdb, gdbserver, gdbsupport: replace many uses of strcmp with streq

Replace all occurrences of:

  strcmp (...) == 0
  strcmp (...) != 0
  !strcmp (...)
  0 == strcmp (...)
  strcmp (...) directly used as a boolean predicate

with the equivalent expression using streq.

This is for consistency (we already use streq as some places in the
testsuite) but also for clarity.  I think that streq is clearer on the
intent than strcmp.  It's also a bit shorter.

Change-Id: Ibbf5261b1872c240bc0c982c147f6a5477275a91
Approved-By: Andrew Burgess <aburgess@redhat.com>