[compiler-rt] Fix src_rep_t_clz and clz_in_sig_frac

This is a follow-up to 910a4bf5.

1. __builtin_clz takes unsigned int, thus for uint16_t
src_rep_t_clz can't use it directly but should subtract 16
(leading 16 bits of the promoted argument are zero).

2. Fix (and simplify) clz_in_sig_frac.

Test plan: ninja check-compiler-rt
(extendhfsf2_test.c and extenddftf2_test.c)