Mark fcontext asm functions as hidden visibility #181

Job	Run time
posix (TOOLSET=gcc COMPILER=g++ CXXSTD=11 Job 0, boost, ubuntu:16.04, g++, gcc, g++, 11)	1m 11s
posix (TOOLSET=gcc COMPILER=g++-5 CXXSTD=11,14,1z Job 1, boost, g++-5, ubuntu:16.04, g++-5, gcc, ...	53s
posix (TOOLSET=gcc COMPILER=g++-6 CXXSTD=11,14,1z Job 2, boost, g++-6, ubuntu:16.04, g++-6, gcc, ...	51s
posix (TOOLSET=gcc COMPILER=g++-7 CXXSTD=11,14,17 Job 3, boost, g++-7, ubuntu:16.04, g++-7, gcc, ...	1m 12s
posix (TOOLSET=clang COMPILER=clang++ CXXSTD=11 Job 4, boost, clang, ubuntu:18.04, clang++, clang...	2m 21s
posix (TOOLSET=clang COMPILER=clang++-4.0 CXXSTD=11, Job 5, boost, clang-4.0 libstdc++-6-dev, ubu...	54s
posix (TOOLSET=clang COMPILER=clang++-5.0 CXXSTD=11, Job 6, boost, clang-5.0 libstdc++-7-dev, ubu...	49s
	8m 11s

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