CMake: Fix arch parsing for oss-fuzz

I'll preface by saying, it's not clear to me why the behavior is
different.

Our qt_run_config_test_architecture function builds a target
executable, and then uses file(STRINGS) to try and parse out some
magic strings that we expect the executable to have (architecture,
abi, etc).

In qmake this was done by matching with a regular expression ending on
a \\0 null byte character.

In CMake, we do a string(FIND) and string(SUBSTRING until the end of
the line) on a *line* returned from file(STRINGS). Notably, I did not
find any regexp syntax provided by CMake to try and match a null byte
character.

Note the docs for file(STRINGS) implies to me that *lines* are
detected by looking for newline characters '\n'. The docs also
mention that binary data in the file is ignored. What's binary
data though?

If you open the executable with a hex editor, at least on macOS, the
strings we are interested in are indeed separated by null byte chars,
not newline chars.

On most platforms file(STRINGS) did end a line on the null byte
character.
Except, for some reason not when building Qt for the oss-fuzz project
with clang under Docker.

Calling message() on one of the lines prints a very long string with
null characters seemingly replaced with semicolons, and of course the
matched architecture string is wrong and fails configuration.

For *some reason*, if I add a "REGEX ==Qt=magic=Qt==" option to the
file(STRINGS) command, the captured output lines don't contain
semicolons even when building for oss-fuzz.

The extracted strings are then correct, and configuration succeeds.

Use the REGEX option workaround, with the aim to quickly fix Qt
building for oss-fuzz for 6.0.1 release.

Pick-to: 6.0
Fixes: QTBUG-89047
Change-Id: Iad11c1090c1187aadd39082f196050bf3290df95
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

cmake/QtBaseConfigureTests.cmake

@@ -42,7 +42,8 @@ function(qt_run_config_test_architecture)
     endif()
     message(STATUS "Extracting architecture info from ${_arch_file}.")
 
-    file(STRINGS "${_arch_file}" _arch_lines LENGTH_MINIMUM 16 LENGTH_MAXIMUM 1024 ENCODING UTF-8)
+    file(STRINGS "${_arch_file}" _arch_lines LENGTH_MINIMUM 16 LENGTH_MAXIMUM 1024 ENCODING UTF-8
+         REGEX "==Qt=magic=Qt==")
 
     foreach (_line ${_arch_lines})
         string(LENGTH "${_line}" lineLength)