[libc][arm][setjmp/longjmp] align jmp_buf to 8B

[nickdesaulniers]

https://github.com/ARM-software/abi-aa/blob/main/clibabi32/clibabi32.rst#setjmph

In deference to VFP, XScale Wireless MMX, and other co-processors manipulating 8-byte aligned types, a jmp_buf must be 8-byte aligned.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (paternity leave) (nickdesaulniers)

https://github.com/ARM-software/abi-aa/blob/main/clibabi32/clibabi32.rst#setjmph

> In deference to VFP, XScale Wireless MMX, and other co-processors manipulating 8-byte aligned types, a jmp_buf must be 8-byte aligned.