libstdc++: Micro-optimize construction of named std::locale

libstdc++: Micro-optimize construction of named std::locale

This shaves about 100ns off the std::locale constructor for named
locales (which is only about 1% of the total time).

Using !*s instead of !strcmp(s, "") doesn't make any difference as GCC
optimizes that already even at -O1. !strcmp(s, "C") is optimized at -O2
so replacing that with s[0] == 'C' && s[1] == '\0' only matters for the
--enable-libstdcxx-debug builds. But !strcmp(s, "POSIX") always makes a
call to strcmp at any optimization level. We make that strcmp call,
maybe several times, for any locale name except for "C" (which will be
matched before we get to the check for "POSIX").

For most targets, locale names begin with a lowercase letter and the
only one that begins with 'P' is "POSIX". Replacing !strcmp(s, "POSIX")
with s[0] == 'P' && !strcmp(s+1, "OSIX") means that we avoid calling
strcmp unless the string really does match "POSIX".

Maybe more importantly, I find is_C_locale(s) easier to read than
strcmp(s, "C") == 0 || strcmp(s, "POSIX") == 0, and !is_C_locale(s)
easier to read than strcmp(s, "C") != 0 && strcmp(s, "POSIX") != 0.

libstdc++-v3/ChangeLog:

	* src/c++98/localename.cc (is_C_locale): New function.
	(locale::locale(const char*)): Use is_C_locale.