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ARM64: Fix LDP code generation.

Thanks to Zhongwei Yao.

(cherry picked from commit 9493acc1a)

When fusing two LDR (STR) instructions to the single LDP (STP)
instruction, the arm64 emitter shifts the offset value to encode the
immediate. In the case when the offset is negative, the resulting field
value exceeds the 7-bit length of the immediate, see [1]. This results
in the invalid instruction decoding.

This patch fixes this by masking the value with the 7-bit-width mask
`0x7f`.

Sergey Kaplun:
* added the description and the test for the problem

[1]: https://developer.arm.com/documentation/ddi0602/2025-03/Base-Instructions/LDP--Load-pair-of-registers-

Part of tarantool/tarantool#11278

Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Signed-off-by: Sergey Kaplun <skaplun@tarantool.org>
(cherry picked from commit d30d1613f)

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