[libc][NFC] Remove headergen for the cacheline size macro.

We want to be able to build and test the string functions in contexts
like that of Fuchsia where LLVM pieces like tablegen are not available.
Since header generation uses tablegen, we are removing the dependency on
headergen here.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D97363

GitOrigin-RevId: 5f0800cc187a006e0424b9fd66985b39525d4478
8 files changed