gdb.trace: Fix string collection for 64-bit platforms.

gdb.trace: Fix string collection for 64-bit platforms.

String collection always used ref32 to fetch the string pointer.  Make it
use gen_fetch instead.

As a side effect, this patch changes dup+const+trace+pop sequence used
for collecting the string's address to a trace_quick opcode.  This
results in a shorter agent expression.

This appeared to work on x86_64 since it's a little-endian platform, and
malloc (used in gdb.trace/collection.exp) returns addresses in low 4GB.
Noticed and tested on s390x-ibm-linux-gnu, also tested on
i686-unknown-linux-gnu and x86_64-unknown-linux-gnu.

gdb/ChangeLog:

	* ax-gdb.c (gen_traced_pop): Use gen_fetch for string collection.