[gdb/testsuite] Add escape_for_host

[gdb/testsuite] Add escape_for_host

In gdb_compile we have:
...
           lappend new_options "ldflags=-Wl,-rpath,\\\$ORIGIN"
...
and we could improve readability by using {} rather than "":
...
           lappend new_options {ldflags=-Wl,-rpath,\$ORIGIN}
...

But rather than manually adding escapes in a string, add a new proc
escape_for_host that care of this for us, allowing us to write:
...
           lappend new_options [escape_for_host {ldflags=-Wl,-rpath,$ORIGIN}]
...

Tested on x86_64-linux.