FTBFS hurd: fixing unsupported platform Hurd

FTBFS hurd: fixing unsupported platform Hurd

On the Hurd platform the build failed in
js/src/jit/AsmJSSignalHandlers.cpp:131 because of unsupported
mode for reading / writing thead state with following message.

 # error "Don't know how to read/write to the thread state via the mcontext_t."

The original patch comes from Richard Brown <rbraun@sceen.net>
https://lists.alioth.debian.org/pipermail/pkg-mozilla-maintainers/2014-March/015565.html

After release of Thunderbird 44 there are additional changes made as the
various configuration files where split into more files.

Gbp-Pq: Topic porting-kfreebsd-hurd
Gbp-Pq: Name FTBFS-hurd-fixing-unsupported-platform-Hurd.patch