Fix missing IPA lib in tspeed.exp in some configurations.

On Ubuntu 14.04 the following failure would be seen when running the
tspeed.exp test on a target that supports fast tracepoints like x86_64:

Target returns error code '.In-process agent library not loaded in
process.  Fast and static tracepoints unavailable.'.
(gdb) FAIL: gdb.trace/tspeed.exp: start trace experiment

This is because the default is to link with --as-needed and the
gdb_compile for the test is using the libs argument instead of shlib which
corrects this issue since 6ebea266fd0a7a56c90db3ab6237ff9f6c919747 by
adding -Wl,--no-as-needed.

This patch fixes the issue by passing the lib as the shlib argument to
gdb_compile.

Tested on Ubuntu 14.04 x86_64.

gdb/testsuite/ChangeLog:

	* gdb.trace/tspeed.exp:  Use shlib instead of libs in gdb_compile
	command.