community/tcl-lib: enable tests & main/tcl: fix rounding problem on x86 (that causes tcl-lib math tests to fail)

One of the complaints about tcl-lib is that it doesn't run its testsuite.

This MR enables the testsuite, and so far, the only failing tests are those that ran on s390x, probably because of its different endianness.

Edit (almost 1 month later): On x86, there is some math rounding problem that requires patching tcl.

The problem seems to be some x86-specific workaround for FPU rounding mode in generic/tclStrToD.c, that apparently isn't needed/has unintended side effects on x86 musl libc.