libffi: Add -mno-compact-eh when compiling for Mips

Fix compilation on Mips. Resolve issues detected here:
    http://autobuild.buildroot.net/results/f0c9db496233f53c26e92294d7305aa4511ec7b3/
    http://autobuild.buildroot.net/results/8968690c248df86b040218867f92b573721e872c/
    http://autobuild.buildroot.net/results/a35db8e6a49d022133b486b12f6a8e40b3c95b6e/



Also remove previous special case handling defined in libffi.mk.

[Thomas: add Signed-off-by from Jérôme inside the patch itself.]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>