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 6ebea266fd 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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-01-20  Antoine Tremblay  <antoine.tremblay@ericsson.com>
+
+	* gdb.trace/tspeed.exp:  Use shlib instead of libs in gdb_compile
+	command.
+
 2016-01-19  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* Makefile.in (DO_RUNTEST): Add --status and update usages.

gdb/testsuite/gdb.trace/tspeed.exp

@@ -21,7 +21,7 @@ set executable $testfile
 set ipalib [get_in_proc_agent]
 
 if { [gdb_compile "$srcdir/$subdir/$srcfile" $binfile \
-	  executable [concat {debug nowarnings c} libs=$ipalib]] != "" } {
+	  executable [concat {debug nowarnings c} shlib=$ipalib]] != "" } {
     untested tspeed.exp
     return -1
 }