libgcc: IA64: don't compile glibc-based unwinder without libc headers

libgcc: IA64: don't compile glibc-based unwinder without libc headers

In --without-headers mode gcc fails to bootstrap on libgcc as:

    /build/build/./gcc/xgcc -B/build/build/./gcc/ ... -Dinhibit_libc -c fde-glibc.c
    ../../../gcc-12-20220403/libgcc/config/ia64/fde-glibc.c:33:10:
        fatal error: stdlib.h: No such file or directory

Most other linux targets are able to build the --without-headers
compiler without additional effort. This change adds IA64 to the fold.

The change drops part of the code that relies on DYNAMIC glibc
section traversal for backtraces.

Tested bootstrap of ia64-unknown-linux-gnu with and without libc
headers present.

libgcc/
	* config/ia64/fde-glibc.c: Make a no-op in inhibit_libc mode.