rpm: let CMake assemble with gcc on el8 (#75)

`-DCMAKE_ASM_COMPILER=.../as` hands libffi's preprocessed `.S` sources
to the bare assembler, which reads CMake's `-D` definitions as
`--defsym` and stops the el8 build:

    Building ASM object .../libffi/.../src/x86/unix64.S.o
    Fatal error: bad defsym; format is --defsym name=value

Without the override CMake takes the ASM compiler from
`CMAKE_C_COMPILER`, the gcc-toolset-11 gcc. Verified with a full build
in the build-deps:el8 image.

Assisted-by: Claude Opus 5.5 (1M context)

Author: sjoelund

rpm/SPECS/openmodelica.spec.tpl

@@ -93,7 +93,7 @@ BuildRequires: libarchive >= 3.3.3
 %{?el8:Requires: gcc-toolset-11-gcc gcc-toolset-11-gcc-c++ gcc-toolset-11-gcc-gfortran}
 %if 0%{?rhel} == 8
 BuildRequires: gcc-toolset-11-gcc gcc-toolset-11-gcc-c++ gcc-toolset-11-gcc-gfortran
-%define devtoolscmakeflags -DCMAKE_C_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/gcc -DCMAKE_CXX_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/g++ -DCMAKE_Fortran_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/gfortran -DCMAKE_ASM_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/as
+%define devtoolscmakeflags -DCMAKE_C_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/gcc -DCMAKE_CXX_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/g++ -DCMAKE_Fortran_COMPILER=/opt/rh/gcc-toolset-11/root/usr/bin/gfortran
 %endif
 
 %if 0%{?omnogui}