[lld] Make -z keep-text-section-prefix recognize .text.split. as a prefix.

".text.split." holds symbols which are split out from functions in
other input sections. For example, with -fsplit-machine-functions,
placing the cold parts in .text.split instead of .text.unlikely mitigates
against poor profile inaccuracy. Techniques such as hugepage remapping can
make conservative decisions at the section granularity.

Differential Revision: https://reviews.llvm.org/D87840