Fix remote-sim.c compilation

The change "make string-like set show commands use std::string
variable" caused remote-sim.c to fail to build.  The issue is that the
code does:

  const std::string &sysroot = gdb_sysroot;
  if (is_target_filename (sysroot))
    sysroot += strlen (TARGET_SYSROOT_PREFIX);

... which isn't valid.

This patch changes this code to use a 'const char *' again, fixing the
build.