maint: pacify GCC 16 -Wuseless-cast

maint: pacify GCC 16 -Wuseless-cast

This mostly either removes casts if they are always no-ops, or
replaces casts (TYPE)(EXPR) with compound literals (TYPE){EXPR}.
Compound literals are a bit safer anyway, as casts are too
powerful in C.
* src/cksum_avx2.c (cksum_avx2):
* src/cksum_avx512.c (cksum_avx512):
* src/cksum_crc.c (cksum_slice8):
* src/cksum_pclmul.c (cksum_pclmul):
* src/cp-hash.c (remember_copied):
* src/numfmt.c (simple_strtod_float):
* src/system.h (x_timestyle_match):
Omit no-op casts.
* src/cp-hash.c (src_to_dest_hash):
* src/dd.c (dd_copy):
* src/df.c (devlist_hash):
* src/env.c (splitbuf_append_byte):
* src/getlimits.c (print_int, main):
* src/ls.c (dev_ino_hash):
* src/truncate.c (do_ftruncate):
Replace casts with compound literals.
* src/factor.c: Ignore -Wuseless-cast, as we include the imported
longlong.h file, which has many of them.
* src/numfmt.c (powerld): 2nd arg is now ptrdiff_t, not int, so
that integers are not silently mishandled in outlandish cases.