boost: fix build on ppc64le host

Fixes:
http://autobuild.buildroot.net/results/57d/57d9f0ea27e5c8ba73002bd1d0b33027f27a3779/
http://autobuild.buildroot.net/results/7c3/7c3133e822c997879fe00923ba0ad7903656c2e1/



bootstrap by default runs ./tools/build/src/engine/build.sh --guess-toolset
to detect what toolchain (compiler variant).  On x86 this returns gcc, but
on the ppc64le gcc112 autobuilder this returns xlcpp causing bootstrap.sh to
get confused and bail out:

./bootstrap.sh ..
Building Boost.Build engine with toolset ... tools/build/src/engine/###
\### No toolset specified. Please use --toolset option.
\###
\### Known toolsets are: acc, borland, cc, como, clang, darwin, gcc, gcc-nocygwin, intel-darwin, intel-linux, intel-win32, kcc, kylix, metrowerks, mipspro, msvc, qcc, pathscale, pgi, sun, sunpro, tru64cxx, vacpp, xlcpp, vc7, vc8, vc9, vc10, vc11, vc12, vc14, vc141, vmsdecc
\###/b2

Fix it by explicitly specifying the gcc toolset mode to bootstrap, similar
to how it was already done for the bjam invocations.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>