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

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.