[binutils-gdb] testsuite: Don't set SYMBOL_PREFIX for x86_64_*_cygwin

[sergiodj+buildbot@redhat.com]

*** TEST RESULTS FOR COMMIT 7ec911b0fb0b27d66604c3f7951c146d94f4acc7 ***

Author: Jon Turney <jon.turney@dronecode.org.uk>
Branch: master
Commit: 7ec911b0fb0b27d66604c3f7951c146d94f4acc7

testsuite: Don't set SYMBOL_PREFIX for x86_64_*_cygwin
Exactly like x86_64-*-mingw, SYMBOL_PREFIX should not be set to "_" for
x86_64_*_cygwin

gdb/testuite/ChangeLog:

	* lib/gdb.exp (gdb_target_symbol_prefix_flags): Don't set
	SYMBOL_PREFIX for x86_64-*-cygwin.