Automatically handle includes in testsuite/lib/

Instead of manually calling lappend_include_file in every testcase
that needs to include a file in testsuite/lib/, handle testsuite/lib/
includes automatically in gdb_compile.

As an example, gdb.base/backtrace.exp is adjusted to no longer
explicitly call lappend_include_file for testsuite/lib/attributes.h.

Tested on x86-64 GNU/Linux with both:

 $ make check RUNTESTFLAGS=" \
     --host_board=local-remote-host-native \
     --target_board=local-remote-host-native \
     HOST_DIR=/tmp/foo/" \
     TESTS="gdb.base/backtrace.exp"

and:

 $ make check TESTS="gdb.base/backtrace.exp"

and confirming that the testcase still compiles and passes cleanly.

Also ran full testsuite on x86-64 GNU/Linux in normal mode.

Approved-by: Kevin Buettner <kevinb@redhat.com>
Change-Id: I5ca77426ea4a753a995c3ad125618c02cd952576