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.

compile_flags.txt

--xc
--Irts
+-Wimplicit
+-include
+rts/include/ghcversion.h
 -Irts/include
--I.hie-bios/stage0/lib
--I_build/stage1/rts/build/include/
+-I_build/stage1/rts/build
+-I_build/stage1/rts/build/include
+-Irts
+-Ilibraries/ghc-internal/include
+-I_build/stage1/libraries/ghc-internal/build/include
+-Ilibraries/ghc-bignum/include
+-I_build/stage1/libraries/ghc-bignum/build/include
+-Wno-unknown-pragmas
+-Wall
+-Wextra
+-Wstrict-prototypes
+-Wmissing-prototypes
+-Wmissing-declarations
+-Winline
+-Wpointer-arith
+-Wmissing-noreturn
+-Wnested-externs
+-Wredundant-decls
+-Wundef
+-DFS_NAMESPACE=rts
+-DCOMPILING_RTS
+-DTHREADED_RTS
+-DDEBUG
+-DDYNAMIC
+-DPROFILING