gnupg: don't prefix assembly level functions with underscore

Fixes http://autobuild.buildroot.net/results/45a0856bafa9f2f7e86e2c063528c2b5b04c08d6



gnupg's configure script defaults to prepending an underscore ('_') to
the assembly level functions, which isn't correct for Linux and causes
linker errors for the archs where it has asm optimizations.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>