Re-enable msabi testing (#436)

Re-enable msabi testing (#436)

* Revert "disable msabi testing for now"

This reverts commit 7b7638eb0eac2adfa72f7ec9f254ba287c9947e2.

* x86: Correct testing for 64-bit x86-64

Since passing -m32 to x86-64 compiler will generate i386 code, we
need to check both __ILP32__ and __i386__ for 64-bit x86-64.

* x86: Check __i386__ instead of i?86-*-* targets

Since passing -m32 to x86-64 compiler will generate i386 code, we need
to check __i386__ instead of i?86-*-* targets for i386 targets.

* i386: Properly passing integer parameters in registers

For thiscall and fastcall, if the paramter passed as 64-bit integer or
struct, all following integer paramters will be passed on stack.

* test: Add ABI_ATTR to callback_code

Add ABI_ATTR to callback_code to properly test different ABIs.