libffi: add patch to fix MIPS support

Building Python 3.x on MIPS with musl fails because the libffi code
uses a "#ifdef linux" test to decide if we're building on Linux or
not. When building with -std=c99, "linux" is not defined, so instead
of including <asm/sgidefs.h>, libffi's code tries to include
<sgidefs.h>, which doesn't exist on musl.

The right fix is to use __linux__, which is POSIX compliant, and
therefore defined even when -std=c99 is used.

Note that glibc and uClibc were not affected because they do provide a
<sgidefs.h> header in addition to the <asm/sgidefs.h> one.

Signed-off-by: Mauro Condarelli <mc5686@mclink.it>
[Thomas: reformat patch with Git, add a better commit log and description.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>