[gdb/testsuite] Handle remote host in escape_for_host

[gdb/testsuite] Handle remote host in escape_for_host

With test-case gdb.arch/ftrace-insn-reloc.exp and host board
local-remote-host-notty and target board native-gdbserver, I run into:
...
FAIL: gdb.arch/ftrace-insn-reloc.exp: IPA loaded
...
due to having:
...
$ readelf -d ftrace-insn-reloc | grep RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: []
...
instead of:
...
$ readelf -d ftrace-insn-reloc | grep RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN]
...

Handle this in escape_for_host.

Tested on x86_64-linux.