[gdb/testsuite] Fix quoting issue in gdb.base/index-cache.exp

[gdb/testsuite] Fix quoting issue in gdb.base/index-cache.exp

For test-case gdb.base/index-cache.exp and remote host, this:
...
lassign [remote_exec host sh "-c \"rm $cache_dir/*.gdb-index\""] ret
...
gives us:
...
Executing on host: sh -c rm /tmp/tmp.m3L7m2AVkL/*.gdb-index    (timeout = 300)
builtin_spawn -ignore SIGHUP sh -c rm /tmp/tmp.m3L7m2AVkL/*.gdb-index^M
rm: missing operand^M
Try 'rm --help' for more information.^M
FAIL: gdb.dwarf2/per-bfd-sharing.exp: couldn't remove files in temporary cache dir
...

Fix this using quote_for_host.  Likewise in gdb.dwarf2/per-bfd-sharing.exp.

Tested on x86_64-linux.