[runtimes] Fix parsing of LIB{CXX,CXXABI,UNWIND}_TEST_PARAMS (#67691)

Since 78d649a4 the recommended way to
pass an executor is to use the _TEST_PARAMS variable, which means we now
pass more complicated value (including ones that may contain multiple
`=`) as part of this variable. However, the `REGEX REPLACE` being used
has greedy matches so everything up to the last = becomes part of the
variable name which results in invalid syntax in the generated lit
config file.

This was noticed due to builder failures for those using the
CrossWinToARMLinux.cmake cache file.

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Co-authored-by: Vladimir Vereschaka <vvereschaka@accesssoftek.com>