When building pkg-config files, compute linker flags with a string
substitution rather than the JOIN generator expression. This ensures
that commas in linker flags don’t get treated as argument separators in
JOIN.

Bug: https://bugs.debian.org/1011294
PiperOrigin-RevId: 450675966
Change-Id: I61eacc46a468bae5ff3dae2b437a564f2f1042c2