gnupg 1.4.2.2: Fix for x86. The assembler code was being compiled

gnupg 1.4.2.2: Fix for x86. The assembler code was being compiled
with underscores prefixed to the functions while the C code was not
expecting the underscores. This was caused by the configure script
assuming that _'s were required when cross-compiling. Turning off
assembler code via a configure option would have fixed this, but
instead I've modified the configure script to actually test for
underscores when cross-compiling - the test is just a compile + nm
which works fine when cross-compiling. Tested on i486, i586 and sh4.