[3.12] gh-110190: Fix ctypes structs with array on Arm (#112604) (#112767)

Set MAX_STRUCT_SIZE to 32 in stgdict.c when on Arm platforms.
This because on Arm platforms structs with at most 4 elements of any
floating point type values can be passed through registers. If the type
is double the maximum size of the struct is 32 bytes.
On x86-64 Linux, it's maximum 16 bytes hence we need to differentiate.

(cherry picked from commit bc68f4a4abcfbea60bb1db1ccadb07613561931c)