misc: improve clangd compile_flags.txt flags

This patch improves the compile_flags.txt config used to power clangd
for the rts C codebase. The flags in the file are sampled & deduped
from a real stage1 build with clang-19 and vastly improves the IDE
accuracy when hacking the rts.

For maximum code coverage under the default settings,
compile_flags.txt defaults to threaded+profiled+dynamic+debug way.
This does not mean profdyn needs to be actually built in _build/stage1
for IDE to work. To activate IDE for other RTS ways, simply remove one
of the -D flags at the end of compile_flags.txt and restart clangd.

(cherry picked from commit 59e0a770)