[gdb] Fix grammar in comments and docs

[gdb] Fix grammar in comments and docs

Fix grammar in some comments and docs:
- machines that doesn't -> machines that don't
- its a -> it's a
- its the -> it's the
- if does its not -> if it does it's not
- one more instructions if doesn't match ->
  one more instruction if it doesn't match
- it's own -> its own
- it's first -> its first
- it's pointer -> its pointer

I also came across "it's performance" in gdb/stubs/*-stub.c in the HP public
domain notice, I've left that alone.

Tested on x86_64-linux.