Build glibc with unwind tables

Without debug symbols or this you cannot have reliable backtraces in gdb. For
instance this is on a Raspberry Pi without the unwind tables:

	$ gcc -x c <(echo 'void abort(); int main() { abort(); }')
	$ gdb ./a.out <<< $'r\nbt'
	...
	#1  0xb6eb2e44 in abort () from /usr/lib/libc.so.6
	#2  0x00000020 in ?? ()
	Backtrace stopped: previous frame identical to this frame (corrupt
	stack?)
	...

Here's with the unwind tables enabled:

	$ gcc -x c <(echo 'void abort(); int main() { abort(); }')
	$ gdb ./a.out <<< $'r\nbt'
	...
	#1  0xb6eade44 in abort () from /usr/lib/libc.so.6
	#2  0x000084e0 in main ()
	...

The upside of the unwind tables compared to the debug symbols is that they don't
take up a lot of space.
Merge pull request #719 from ypsu/master

Build glibc with unwind tables