A 64-bit signal frame is 1552 bytes; rounded up and with the 512 byte
red zone added it needs 2096 bytes, more than MINSIGSTKSZ (2048), so a
handler on a minimum size alternate stack had its frame start 48 bytes
below the stack. The red zone only exists below the interrupted code's
stack pointer, so skip it only when the frame goes on that stack.
A handler on a MINSIGSTKSZ alternate stack overwrote the 48 bytes below
it; the frame stays within the stack now.
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