webkitgtk24: fix MIPS64 n64 ABI detection

webkitgtk24 is failing to compile for MIPS64 n64 because the ABI is not
detected correctly. It causes failures like these ones:

./Source/JavaScriptCore/runtime/JSCJSValueInlines.h:201:53: error: cast
from 'JSC::JSCell*' to 'int32_t {aka int}' loses precision
[-fpermissive]

[snip]

./Source/WTF/wtf/StdLibExtras.h:137:5: error: static assertion failed:
bitwise_cast size of FromType and ToType must be equal!

Bug report:

  https://bugs.webkit.org/show_bug.cgi?id=145113

Upstream patch:

  http://trac.webkit.org/changeset/185863

Fixes:

  http://autobuild.buildroot.net/results/0d5/0d56a5bf6e92e7344dcee7acb85e176198f703e7/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>