Fix incorrect name for x86 platform's gdb

gdbclient looks for 'x86_64-linux-androideabi-gdb' whereas the prebuilts
have gdb as 'x86_64-linux-android-gdb'

$ cd $ANDROID_BUILD_TOP
$ find prebuilts -iname *x86*gdb
prebuilts/gcc/linux-x86/x86/x86_64-linux-android-4.9/bin/x86_64-linux-android-gdb
prebuilts/gcc/linux-x86/x86/x86_64-linux-android-4.8/bin/x86_64-linux-android-gdb
prebuilts/gcc/darwin-x86/x86/x86_64-linux-android-4.9/bin/x86_64-linux-android-gdb
prebuilts/gcc/darwin-x86/x86/x86_64-linux-android-4.8/bin/x86_64-linux-android-gdb

Change-Id: Ib940ecd6f345ea501da79834168b91cc0f6e5a96