Remove a flag from riscv64 that was copied from arm64.

This causes us to align the text section to 4KiB, not just the data
section. From the bug that added this for arm64, there was an admission
that it costs 17MiB across the tree, and a not entirely convincing claim
of a performance improvement (undermined by the fact that "some stuff
was faster, some stuff was slower"). See http://b/150506341 for that.

Anyway, let's assume we don't want this for riscv64 for now.

Test: `mm -j` in bionic and readelf on the resulting libtest_empty.so
Change-Id: I0ed592a97ef705175191d9c66271a599af4eb18f