[libc][arm][setjmp/longjmp] define `__aeabi_JMP_BUF_SIZE`

[nickdesaulniers]

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

The link-time constant __aeabi_JMP_BUF_SIZE gives the actual size of a target system jmp_buf measured in 8-byte double-words.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (paternity leave) (nickdesaulniers)

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

> The link-time constant __aeabi_JMP_BUF_SIZE gives the actual size of a target system jmp_buf measured in 8-byte double-words.