arm64: fix padding computation in struct ucontext

The expression to compute the padding needed to fill the uc_sigmask field
to 1024 bits actually computes the padding needed for 1080 bits.
Fortunately, due to the 16-byte alignment of the following field
(uc_mcontext) the definition in glibc contains enough bytes of padding
after uc_sigmask so that the overall offsets and size match in both
definitions.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>