test: simplify T360-symbol-hiding, use nm instead of objdump

After yet another variation in objdump output caused this test to fail
(on a Debian port, no less), I decided whatever putative benefit we
get from looking at the object files instead of the library isn't
worth the maintenence headache.

This version uses nm -P. nm -P should be portable, and fixed format.
It purposely doesn't use the -D argument, since that is non-POSIX and
nm on GNU/Linux seems do the right thing without it.

It still won't work out of the box on e.g. Mac OS/X. I think the right
thing to do there is to move some more configuration information into
sh.config.

(cherry picked from commit c34d6bad0f)

test/T360-symbol-hiding.sh

@@ -26,7 +26,7 @@ test_begin_subtest 'checking output'
 test_expect_equal "$result" "$output"
 
 test_begin_subtest 'comparing existing to exported symbols'
-objdump -t $TEST_DIRECTORY/../lib/*.o | awk '$4 == ".text" && $6 ~ "^notmuch" {print $6}' | sort | uniq > ACTUAL
+nm -P $TEST_DIRECTORY/../lib/libnotmuch.so | awk '$2 == "T" && $1 ~ "^notmuch" {print $1}' | sort | uniq > ACTUAL
 sed -n 's/[[:blank:]]*\(notmuch_[^;]*\);/\1/p' $TEST_DIRECTORY/../notmuch.sym | sort | uniq > EXPORTED
 test_expect_equal_file EXPORTED ACTUAL