x86_64 porting changes.
Use INT2PTR and PTR2INT to handle the
stashing of the PerlClientApi pointer in a Perl scalar. This is
necessary because, despite all documentation to the contrary, an
I32 is not 64-bit capable on all 64-bit machines.
Also, the hints file now looks for x86_64 in the architecture
name and if it finds it, then const_char='const char' rather
than the default of 'char'.