[gdb/testsuite] Allow override of ASAN_OPTIONS in lib/gdb.exp

Use set_sanitizer_default for ASAN_OPTIONS in lib/gdb.exp.

This allows us to override the default detect_leaks=0 setting, by manually
doing:
...
$ export ASAN_OPTIONS=detect_leaks=1
$ make check
...

Tested on x86_64-linux, by building with -fsanitize=address and running
test-case gdb.dwarf2/gdb-add-index.exp with and without
"export ASAN_OPTIONS=detect_leaks=1".