[sanitizer] Use consistent checks for XDR

sanitizer_platform_limits_posix.h defines `__sanitizer_XDR ` if `SANITIZER_LINUX && !SANITIZER_ANDROID`, but sanitizer_platform_limits_posix.cpp tries to check it if `HAVE_RPC_XDR_H`. This coincidentally works because macOS has a broken <rpc/xdr.h> which causes `HAVE_RPC_XDR_H` to be 0, but if <rpc/xdr.h> is fixed then clang fails to compile on macOS. Restore the platform checks so that <rpc/xdr.h> can be fixed on macOS.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D130060