[Support] Fix build on illumos

D116366 added a call to madvise() in Path.inc. Unfortunately,
Illumos does not declare this function if _XOPEN_SOURCE is defined
(which it is by default) and we need to provide the declaration
manually. This is the same workaround used in sanitizers:
https://github.com/llvm/llvm-project/blob/ee423d93ead39e94c2970b3cc7ef6e6faa75d10b/compiler-rt/lib/sanitizer_common/sanitizer_solaris.cpp#L77-L85

Differential Revision: https://reviews.llvm.org/D119695