Fix tparm kludge

This just defines a constant to whichever tparm implementation we're
using (either the actual, working one the system provides, or our
kludge to paper over Solaris' inadequacies).

This means that there won't be so much ping-ponging of what "tparm"
stands for. "tparm" is the system's function. Only we don't use it,
just like we don't use wcstod directly.

Fixes #8780

src/fallback.cpp

@@ -44,16 +44,10 @@
 #include "fallback.h"  // IWYU pragma: keep
 
 #if defined(TPARM_SOLARIS_KLUDGE)
-#undef tparm
-
 char *tparm_solaris_kludge(char *str, long p1, long p2, long p3, long p4, long p5, long p6, long p7,
                            long p8, long p9) {
     return tparm(str, p1, p2, p3, p4, p5, p6, p7, p8, p9);
 }
-
-// Re-defining just to make sure nothing breaks further down in this file.
-#define tparm tparm_solaris_kludge
-
 #endif
 
 int fish_mkstemp_cloexec(char *name_template) {