[patch] libstdc++/69581 Don't define guard macros when doing #include_next in math.h and stdlib.h

[Jonathan Wakely]

The new <math.h> and <stdlib.h> wrapper headers should not define
their guard macros when being included in "pass-through" mode, i.e.
when _GLIBCXX_INCLUDE_NEXT_C_HEADERS is defined. This patch allows the
wrapper to pass-through as many layers of wrapper headers as needed
until the real C library header is found.

Tested powerpc64-linux, committed to trunk.

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