[lldb] Fix build on older FreeBSD

Commit 392d9eb0 added a dependency on
FPE_FLTIDO which was only defined in FreeBSD main on May 19, 2022 and
is not in all supported releases. Just define it if it's missing as we
could use a debugger compiled on an older system to debug a newer one.

Reviewed by: DavidSpickett, emaste, dim

Differential Revision: https://reviews.llvm.org/D147300