[openmp] Support building for armv7 Windows with mingw tools

This does things in the same way as
D137168 / a3567824 and
D101173 / 4fb0aaf0 did for aarch64.

This adds a C implementation of __kmp_invoke_microtask in the same
way as the fallback C implementation in z_Linux_util.cpp.

Both the existing C fallback used on arm linux, and this one added here,
fail test/misc_bugs/many-microtask-args.c similarly (which could be
considered as an XFAIL).

Differential Revision: https://reviews.llvm.org/D138689