[PowerPC] Add clang -msvr4-struct-return for 32-bit ELF

Summary:

Change the default ABI to be compatible with GCC. For 32-bit ELF
targets other than Linux, Clang now returns small structs in
registers r3/r4. This affects FreeBSD, NetBSD, OpenBSD. There is no
change for 32-bit Linux, where Clang continues to return all structs
in memory.

Add clang options -maix-struct-return (to return structs in memory)
and -msvr4-struct-return (to return structs in registers) to be
compatible with gcc. These options are only for PPC32; reject them on
PPC64 and other targets. The options are like -fpcc-struct-return and
-freg-struct-return for X86_32, and use similar code.

To actually return a struct in registers, coerce it to an integer of
the same size. LLVM may optimize the code to remove unnecessary
accesses to memory, and will return i32 in r3 or i64 in r3:r4.

Fixes PR#40736

Patch by George Koehler!

Reviewed By: jhibbits, nemanjai
Differential Revision: https://reviews.llvm.org/D73290