buildtools: make experimental symbol check lighter

buildtools: make experimental symbol check lighter

Dumping every object file for every symbol is too heavy.
Use a temporary storage.

Before:
$ rm -rf master && make defconfig O=master
$ time make EXTRA_CFLAGS=-g O=master
[...]
real	2m24.063s
user	1m16.985s
sys	1m46.372s

After:
$ rm -rf master && make defconfig O=master
$ time make EXTRA_CFLAGS=-g O=master
[...]
real	1m37.110s
user	0m49.417s
sys	0m51.803s

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>