Cygwin compile fixes

Summary: Small fixes and workaround for compiling on cygwin

mainly missing symbols, using windows apis for functionality, and a few workarounds for C library differences
such as MAX and isinf, isnan (which are all easily available in c++)

Reviewed By: @sgolemon

Differential Revision: D1414208
Fixing OSS Bug #3130

Summary: An interaction between the simplifier, guard relaxation, and member instructions
was causing a Str to be relaxed to a Cell despite a ConcatStrStr consuming the
string.

Reviewed By: @swtaarrs

Differential Revision: D1467017