rts: Align the_gc_thread to 64 bytes
rts: Align the_gc_thread to 64 bytes

In a previous attempt (c6cc93bca69abc258513af8cf2370b14e70fd8fb) I had
tried aligning to 8 bytes under the assumption that the problem was that
the_gc_thread, a StgWord8[], wasn't being aligned to 8-bytes as the
gc_thread struct would expect. However, we actually need even stronger
alignment due to the alignment attribute attached to gen_workspace,
which claims it should be aligned to a 64-byte boundary.

This fixes #15482.

Reviewers: erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: #15482

Differential Revision: https://phabricator.haskell.org/D5052

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc