Extended double precision support in flang

The flang compiler allows compiling Fortran code which uses extended double precision intrinsic functions, like `abs`. However when linking with the flang runtime library, the required symbols are not available.

Tested on package https://github.com/fortran-lang/test-drive v0.6.0 with flang 21.1.8 from conda-forge on Windows.

````
LINK: command "D:\bld\test-drive-pkg_1775033357038\_build_env\Library\bin\lld-link.exe /nologo test\CMakeFiles\test-drive-tester.dir\main.f90.obj test\CMakeFiles\test-drive-tester.dir\test_check.F90.obj test\CMakeFiles\test-drive-tester.dir\test_select.F90.obj /out:test\test-drive-tester.exe /implib:test\test-drive-tester.lib /pdb:test-drive-tester.exe.dbg /version:0.0 /machine:x64 -Wl,-defaultlib:D:/bld/test-drive-pkg_1775033357038/_build_env/lib/clang/21/lib/windows/clang_rt.builtins-x86_64.lib /INCREMENTAL:NO /subsystem:console test-drive.lib -libpath:D:/bld/test-drive-pkg_1775033357038/_build_env/Library/lib/clang/21/lib/windows -libpath:D:/bld/test-drive-pkg_1775033357038/_build_env/Library/lib -libpath:D:/bld/test-drive-pkg_1775033357038/_build_env/Library/lib/clang/21/lib/x86_64-pc-windows-msvc /MANIFEST:EMBED,ID=1" failed (exit code 1) with the following output:
lld-link: warning: ignoring unknown argument '-Wl,-defaultlib:D:/bld/test-drive-pkg_1775033357038/_build_env/lib/clang/21/lib/windows/clang_rt.builtins-x86_64.lib'

lld-link: error: undefined symbol: __extendxftf2
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced 12 more times

lld-link: error: undefined symbol: __trunctfxf2
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_float_xdp)
>>> referenced 7 more times

lld-link: error: undefined symbol: _FortranACAbsF128
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_complex_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_complex_xdp)
>>> referenced by test-drive.lib(testdrive.F90.obj):(_QMtestdrivePcheck_complex_xdp)
>>> referenced 2 more times
````

````
conda list flang
flang: 21.1.8-h1b5488d_0 conda-forge
flang-rt_win-64: 21.1.8-h57928b3_0 conda-forge
flang_impl_win-64: 21.1.8-h719f0c7_0 conda-forge
flang_win-64: 21.1.8-h719f0c7_0 conda-forge
````

````
conda info
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active env location : D:\Miniforge
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populated config files : D:\Miniforge\.condarc
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platform : win-64
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administrator : True
netrc file : None
offline mode : False
````