Introduce CONFIG_LIBC_DISABLE_HEXSTR_TO_FLOAT to remove support for
parsing hexadecimal floating-point constants (C99 %a) in strtod(),
strtof(), strtold(), and sscanf("%a"). This feature is rarely used in
embedded systems and its removal saves significant flash space. Decimal
float parsing remains unaffected.
Signed-off-by: Peter van der Perk <peter.vanderperk@nxp.com>