package/libunwind: fix build failure due to asm()

package/libunwind: fix build failure due to asm()

The gcc documentation [1] suggest to use __asm__ instead of asm.

Fixes:
http://autobuild.buildroot.net/results/3ef/3efe156b6494e4392b6c31de447ee2c72acc1a53

[1] https://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html#Alternate-Keywords

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>