[compiler-rt] [test] Add an .exe suffix for unit test executables on MinGW

Previously, an .exe suffix was only added for MSVC configurations.

In practice, an .exe suffix is added implicitly by MinGW toolchains
if the output is a suffixless file name. However this can cause lots
of subtle build system confusion, when it's not generating the file it
expected.

Differential Revision: https://reviews.llvm.org/D149029