folly: attribute-aligned-arg must now be constant (not enum)

folly: attribute-aligned-arg must now be constant (not enum)

Summary:
With gcc-4.8.[01], the argument to __attribute__((__aligned__(...)))
must be a literal;  using an enum member evokes this:

./folly/MPMCQueue.h:341:63: error: requested alignment is not an \
integer constant
size_t capacity_ FOLLY_ON_NEXT_CACHE_LINE;

Changing the enum member to this made no difference:

static constexpr size_t kFalseSharingRange = 64;

Test Plan: compile tao and run test suite

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D926221

@override-unit-failures