Fix build_configs after CHPL_TARGET_COMPILER=llvm change

[mppf]

This PR makes adjustments to build_configs to work with the changes
from PR #17800. In particular, that PR changed make for LLVM
configurations to no longer build the runtime twice (once for LLVM and
once for the current C compiler). But the build_configs module build
was relying on this behavior.

• Set CHPL_TARGET_COMPILER always in the build_configs setenv scripts
• Build LLVM as the CHPL_TARGET_COMPILER separately
• do so for EX, x86, and arm builds

Note that setting CHPL_TARGET_COMPILER on a PrgEnv system used to
result in an error but no longer does. I do not know when that changed
but it was at least not an error at the time of the last release (or at
least not an error to run printchplenv). Now it is necessary to set it in
order to request the C backend on such systems when the LLVM backend is
available.

• checked that the following command functions as expected

util/buildRelease/test_rpm_module.bash  https://github.com/mppf/chapel fix-build-configs-17800

Reviewed by @gbtitus and @Maxrimus - thanks!

[gbtitus]

Not doing a module build to check, but visually and based on my understanding of how the role of CHPL_TARGET_COMPILER has changed for PrgEnv-based systems, this looks good to me.