[Flang/Flang-RT] Fix OldUnit tests on Windows (#150734)

Flang and Flang-RT have two flavours of unittests: 
1. GTest unittests, using lit's `lit.formats.GoogleTest` format ending
with `Tests${CMAKE_EXECUTABLE_SUFFIX}`
2. "non-GTest" or "OldUnit" unittests, a plain executable ending with
`.test${CMAKE_EXECUTABLE_SUFFIX}`

Both executables are emitted into the same unittests/ subdirectory. When
running ...
1. `tests/Unit/lit.cfg.py`, only considers executable ending with
`Tests` (or `Tests.exe` on Windows), hence skips the non-GTest tests.
2. `tests/NonGtestUnit/lit.cfg.py` considers all tests ending with
`.test` or `.exe`. On Windows, The GTest unitests also end with `.exe`.

In Flang-RT, `.exe` is considered an extension for non-GTest unitests
which causes tests such as Flang's `RuntimeTests.exe` to be executed for
both on Windows. This particular test includes a file write test, using
a hard-coded filename `ucsfile`. If the two instances are executed
concurrently, they might interfere with each other reading/writing
`ucsfile` which results in a flaky test.

This patch avoids the redundant execution by requiring the suffix
`.test.exe` on Windows. lit has to be modified because it uses
`os.path.splitext` the extract the extension, which would only recognize
the last component. It was changed from the orginal `endswith` in
c865abe747aa72192f02ebfdcabe730f2553e42f
for unknown reasons.

In Flang, `.exe` is not considered a suffix for non-GTest unittests and
hence they are not run at all. Fixing by also added `.test.exe` as valid
suffix, like with Flang-RT.

Unfortunately, the ` Evaluate/real.test.exe` test was failing on
Windows:
```
FAIL: flang-OldUnit :: Evaluate/real.test.exe (3592 of 3592)
******************** TEST 'flang-OldUnit :: Evaluate/real.test.exe' FAILED ********************
..\_src\flang\unittests\Evaluate\real.cpp:511: FAIL: FlagsToBits(prod.flags) == 0x18, not 0x10
        0 0x800001 * 0xbf7ffffe
..\_src\flang\unittests\Evaluate\real.cpp:511: FAIL: FlagsToBits(prod.flags) == 0x18, not 0x10
        0 0x800001 * 0x3f7ffffe
..\_src\flang\unittests\Evaluate\real.cpp:511: FAIL: FlagsToBits(prod.flags) == 0x18, not 0x10
        0 0x80800001 * 0xbf7ffffe
..\_src\flang\unittests\Evaluate\real.cpp:511: FAIL: FlagsToBits(prod.flags) == 0x18, not 0x10
        0 0x80800001 * 0x3f7ffffe
...
```
This is due to the `__x86_64__` macro not being set by Microsoft's
cl.exe and hence floating point status flags not being read out. The
equivalent macro for Microsofts compiler is `_M_X64` (or `_M_X64`).