[MSAN][MIPS] Fix fork.cc test on MIPS

Summary: For platforms which support slow unwinder only, we restrict the store context size to 1, basically only storing the current pc. We do this because the slow unwinder which is based on libunwind is not async signal safe and causes random freezes in forking applications as well as in signal handlers.

Reviewed by eugenis.
Differential: D23107

llvm-svn: 289027