Solaris Intel 10 supports 64-bit programs as well as Solaris sparc do. While GCC provided by sunfreeware or cleanly build from sources doesn't enable 64-bit mode by default, Sun provides package SUNWgcc, which contains patched gcc 3.4.6, which supports 64-bit mode.
However, Tcl configure doesn't know about ability to build 64-bit version of Tcl on Solaris Intel 10 (and above) platform. It explicitely test for output of Solaris isainfo utility to be equal to "sparcv9 sparc", while "amd64 i386" is 64-bit platform as well.
Attached patch made against configure script of tcl8.4.18 fixes this problem (problem still exists in tcl8.5.6)