Currently, one can pass extra parameters to builds of runtimes
for specific targets, by setting them as e.g.
RUNTIMES_<targetname>_CMAKE_CXX_FLAGS. When not setting
LLVM_RUNTIME_TARGETS (or when it is set to "default"), no flags
are propagated in this way, but one can pass a set of cmake flags
to set via RUNTIMES_CMAKE_ARGS though.
While RUNTIMES_CMAKE_ARGS gives almost the same abilities, it's
slightly more cumbersome less obvious. (This flag is only available
for the runtimes, but not for the builtins.)
Thus, in addition to that existing flag, allow using the same pattern
of prefixing variables with "RUNTIMES_<targetname>_" and
"BUILTINS_<targetname>_", with targetname set to "default".