gh-124111: Only set TCLSH_NATIVE for AMD64/ARM64 (GH-149443)

The Tcl 9 makefile.vc now uses TCLSH_NATIVE during the build process,
not just the installation.  We had been setting it to the installed
location of the x86 tclsh.exe, which does not yet exist when the x86
build process needs it.  That build doesn't actually need TCLSH_NATIVE,
though (there's a check specifically allowing TCLSH to be used if
MACHINE is IX86 and TCLSH_NATIVE is undefined), so don't set it.