Fix gdb.compile/compile.exp shlib regression

Fix gdb.compile/compile.exp shlib regression

Commit 6e774b13c3b8 (Make ftrace tests work with remote targets) made
a few gdb.compile/compile.exp tests disappear:

 -PASS: gdb.compile/compile.exp: call shared library function
 -PASS: gdb.compile/compile.exp: expect 1
 -PASS: gdb.compile/compile.exp: modify shared library variable
 -PASS: gdb.compile/compile.exp: expect 15

This is because the test uses ldflags instead of using the shlib
option, so it misses linking with -rpath, resulting in:

 (gdb) run
 Starting program: .../compile/compile-shlib
 .../compile/compile-shlib: error while loading shared libraries: compile-shlib.so: cannot open shared object file: No such file or directory
 [Inferior 1 (process 18014) exited with code 0177]

We were missing a gdb_load_shlibs call, which is needed for remote
testing.

gdb/testsuite/ChangeLog:
2015-04-07  Pedro Alves  <palves@redhat.com>

	* gdb.compile/compile.exp: Use gdb_compile with "shlib=" option
	instead of build_executable.  Use gdb_load_shlibs.