[libc++] Future-proof generate_feature_test_macro_components.py against long names.

`__cpp_lib_default_template_type_for_algorithm_values` is 52 characters long,
which is enough to reduce the multiplier to less-than-zero, producing an empty
string between the name of the macro and its numeric value. Ensure there's
always a space between the name of the macro and its value.

Differential Revision: https://reviews.llvm.org/D98869