Fix the predefined exponent limit macros for the 16-bit IEEE format.

The magnitude range of normalized _Float16 is 2^-14 (~6e-5) to (2-2^-10)*2^15 (65504). You might think, then, that the code is correct to defne FLT16_MIN_EXP and FLT16_MAX_EXP to be -14 and 15 respectively. However, for some reason the C specification actually specifies a bias for these macros:

C11 5.2.4.2.2: - minimum negative integer such that FLT_RADIX raised to one less than that power is a normalized floating-point number, e_min: FLT_MIN_EXP DBL_MIN_EXP LDBL_MIN_EXP - maximum integer such that FLT_RADIX raised to one less than that power is a representable finite floating-point number, e_max: FLT_MAX_EXP DBL_MAX_EXP LDBL_MAX_EXP

FLT16_MIN_EXP and FLT16_MAX_EXP should clearly be biased the same way, and other compilers do in fact do so, as do our OpenCL headers for `half`.

Additionally, FLT16_MIN_10_EXP is just wrong.