Otherwise Classic Flang's CMakeLists.txt will be unable to find llvm-lit and
cannot generate test-related goals, e.g. `check-all`.

Also fix a typo in build-flang.sh that prevented the option from being passed
to CMake correctly.

…e tree

Support the LLVM_MAIN_SRC_DIR variable which, if set to the LLVM source tree,
allows CMake to locate llvm-lit and generate the "check-libpgmath" goal. Also
update build-flang.sh to use the variable when building libpgmath.

Also modified helpers in runtime/libpgmath/cmake/modules/LibmathUtils.cmake,
e.g. libpgmath_say, libpgmath_warning_say, to print all given arguments instead
of only the first named argument. This allows callers to break long strings
into multiple lines, resulting in better-formatted code.