Various rts fixes for issues spotted by UndefinedBehaviorSanitizer

rts: ensure gc_thread/gen_workspace is allocated with proper alignment

gc_thread/gen_workspace are required to be aligned by 64 bytes. However, this property has not been properly enforced before, and numerous alignment violations at runtime has been caught by UndefinedBehaviorSanitizer that look like:

rts/sm/GC.c:1167:8: runtime error: member access within misaligned address 0x0000027a3390 for type 'gc_thread' (aka 'struct gc_thread_'), which requires 64 byte alignment
0x0000027a3390: note: pointer points here
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/sm/GC.c:1167:8

rts/sm/GC.c:1184:13: runtime error: member access within misaligned address 0x0000027a3450 for type 'gen_workspace' (aka 'struct gen_workspace_'), which requires 64 byte alignment
0x0000027a3450: note: pointer points here
 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/sm/GC.c:1184:13

This patch fixes the gc_thread/gen_workspace misalignment issue by explicitly allocating them with alignment constraint.

rts: fix an unaligned load in nonmoving gc

This patch fixes an unaligned load in nonmoving gc by ensuring the closure address is properly untagged first before attempting to prefetch its header. The unaligned load is reported by UndefinedBehaviorSanitizer:

rts/sm/NonMovingMark.c:921:9: runtime error: member access within misaligned address 0x0042005f3a71 for type 'StgClosure' (aka 'struct StgClosure_'), which requires 8 byte alignment
0x0042005f3a71: note: pointer points here
 00 00 00  98 43 13 8e 12 7f 00 00  50 3c 5f 00 42 00 00 00  58 17 b7 92 12 7f 00 00  89 cb 5e 00 42
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/sm/NonMovingMark.c:921:9

This issue had previously gone unnoticed since it didn't really harm runtime correctness, the invalid header address directly loaded from a tagged pointer is only used as prefetch address and will not cause segfaults. However, it still should be corrected because the prefetch would be rendered useless by this issue, and untagging only involves a single bitwise operation without memory access so it's cheap enough to add.

rts: use __builtin_offsetof to implement STG_FIELD_OFFSET

This patch fixes the STG_FIELD_OFFSET macro definition by using __builtin_offsetof, which is what gcc/clang uses to implement offsetof in standard C. The previous definition that uses NULL pointer involves subtle undefined behavior in C and thus reported by UndefinedBehaviorSanitizer as well:

rts/Capability.h:243:58: runtime error: member access within null pointer of type 'Capability' (aka 'struct Capability_')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/Capability.h:243:58