Do not define tgmath.h fmaxmag, fminmag macros for C2X (bug 28397)

C2X does not include fmaxmag and fminmag.  When I updated feature test
macro handling accordingly (commit
858045ad, "Update floating-point
feature test macro handling for C2X", included in 2.34), I missed
updating tgmath.h so it doesn't define the corresponding type-generic
macros unless __STDC_WANT_IEC_60559_BFP_EXT__ is defined; I've now
reported this as bug 28397.  Adjust the conditionals in tgmath.h
accordingly.

Tested for x86_64.