Fix number formatting function casting

Cast number formatting functions to functions with exact argument type
in hidrd_num_to_alt_str, instead of always casting to functions with
unsigned argument.

This fixes num_test failures on ARM and Sparc as reported by Klee
Dienes:

    The key symptom was that the promotion path for int8 was getting
    mauled by the casts of the functions to hidrd_num_to_alt_str.

Test output would be something like this:

    num_test.c:322: Unexpected formatting result: 128 instead of -128

Based on the fix suggested by him.