Change macro GUIDEDLL_EXPORTS to KMP_DYNAMIC_LIB

A while back, Hal suggested updating the GUIDEDLL_EXPORTS macro guard to 
a more descriptive name.  It represents a dynamic library build so 
KMP_DYNAMIC_LIB is a more suitable name.

Differential Revision: http://reviews.llvm.org/D9899

llvm-svn: 238221