[BPF] Return small aggregates directly in registers (#206876)

Previously the BPF ABI always returned aggregate (struct/union) types
indirectly through an sret pointer, regardless of size. This is
inconsistent with how classifyArgumentType() already passes small
aggregates: arguments up to 128 bits are coerced into one or two 64-bit
registers, while only larger aggregates use an indirect reference.

Make classifyReturnType() mirror that convention by factoring the shared
aggregate handling into a classifyAggregateType() helper used by both:

  - empty aggregates (0 bits) are ignored;
- aggregates up to 64 bits are returned directly, coerced to an integer
of the padded size;
  - aggregates of 65..128 bits are returned directly as [2 x i64];
  - aggregates larger than 128 bits are returned indirectly via sret.

This keeps each returned value within the backend's two-register return
convention and avoids an unnecessary memory round-trip for small
structs.

This also aligns BPF with the general-purpose C ABIs of other targets:
both x86-64 (System V, RAX:RDX) and AArch64 (AAPCS64, X0:X1) return
aggregates up to 16 bytes in a pair of registers and only fall back to
an indirect sret pointer for larger ones.

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>