Depending on the uClibc-specific, internal __STDIO_PUTC_MACRO for
detecting macro-ized *putc* services is non-portable, unlikely to work
with MUSL libc or any other libc variants.

Use a more robust approach by directly detecting the definition of
macros for those services. This change also enables non-wrapped access
to such calls, which was not available until then (__STD() calls were
routed to the wrapper regardless).