Exclude unused builtins from Core file

[dc-mak]

The following are added regardless of whether they are called or not, and it makes the output messy.

-- C function types
ffs_proxy: 'signed int (signed int)'
ffsl_proxy: 'signed int (signed long)'
ffsll_proxy: 'signed int (signed long_long)'
ctz_proxy: 'signed int (unsigned int)'
bswap16_proxy: 'uint16_t (uint16_t)'
bswap32_proxy: 'uint32_t (uint32_t)'
bswap64_proxy: 'uint64_t (uint64_t)'
unreachable_proxy: 'void ()'
__builtin_ctzl: 'signed int (unsigned long)'
__builtin_ctzll: 'signed int (unsigned long_long)'
tester: 'void (struct cluster_t*)'
-- Fun map
proc __builtin_ffs (loaded integer)

proc __builtin_ffsl (loaded integer)

proc __builtin_ffsll (loaded integer)

proc __builtin_ctz (loaded integer)

proc __builtin_ctzl (loaded integer)

proc __builtin_ctzll (loaded integer)

proc __builtin_bswap16 (loaded integer)

proc __builtin_bswap32 (loaded integer)

proc __builtin_bswap64 (loaded integer)

proc __builtin_unreachable ()

After this is done, the CN code below needs to be updated as well:
https://github.com/rems-project/cn/blob/7244b9f9c63d877326ee76394815c1eda886cb73/lib/check.ml#L2686-L2719