Enable building libc++experimental on all arches with C++11 support

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Summary

The libc++experimental library was added recently, but one part of it
(std::experimental::filesystem) contains a number of flawed static
assertion tests, which assume time_t is always 64 bit on LP64
systems, and 32 bit otherwise.

Change the preprocessor guards to test for TIME_T_64BIT instead, which
we can then define appropriately for the platform in the Makefile.

Not that for -CURRENT, as of recently, the only arch left with 32 bit
time_t is i386. If this ever gets MFC'd, that would also include 32 bit
PowerPC, IIRC.

Test Plan

I would like somebody who saw errors earlier, on e.g. arm or powerpc,
check if this now builds correctly. :)

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Abandoning for now, upstream has fixed this in a different way, and I'm going to merge 5.0.0 soon enough anyway.