Fix build errors in mono-context.c on ppc64el

This PR is a follow up to PR 1409, in order to continue working on the pending requested changes (regarding commit "PowerPC64 ELFv2 ABI: cases for in-register parameter passing... ").

I mention ppc64el on the PR title, but AFAICT it is generally applicable to powerpc.

For reference, with it applied, this is the result of make check in mono/tests, to be worked on.

	416 test(s) passed. 11 test(s) did not pass.

	Failed tests:

	bug-481403.exe
	ckfiniteTest.exe
	even-odd.exe
	monitor.exe
	pinvoke11.exe
	pinvoke17.exe
	pinvoke2.exe
	pinvoke3.exe
	pinvoke.exe
	vararg.exe
	winx64structs.exe