ulalaca: Don't redefine char16_t and char32_t

[matt335672]

See #3681

On MacOS, stdint.h is provided by the compiler for C, and by the SDK for C++.

OSX 14.4 appears to define char16_t and char32_t within stdint.h for C++. Defining them again results in:

../common/arch.h:53:24: error: cannot combine with previous 'type-name' declaration specifier
typedef uint_least16_t char16_t;
                       ^
../common/arch.h:53:1: warning: typedef requires a name [-Wmissing-declarations]
typedef uint_least16_t char16_t;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../common/arch.h:54:24: error: cannot combine with previous 'type-name' declaration specifier
typedef uint_least32_t char32_t;
                       ^
../common/arch.h:54:1: warning: typedef requires a name [-Wmissing-declarations]
typedef uint_least32_t char32_t;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~