Clearing of instruction cache

[Marc Nieper-Wißkirchen]

I have noticed that on some targets, like ARM, where it is important to clear the instruction cache before calling jitted code, the jit_flush operation is a no-op when GNU lightning is not compiled with GCC:

https://git.savannah.gnu.org/cgit/lightning.git/tree/lib/jit_arm.c#n2008

This cannot be correct unless GNU lightning is not supposed to be compiled with a different compiler.

It would probably be better to have a check for libgcc in `configure.ac' (e.g. by using `AC_LIB_HAVE_LINKFLAGS' from [1]) instead so that only `libgcc' would be needed and not `gcc' as the C compiler.

Thanks,

Marc

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[1] https://www.gnu.org/software/gnulib/manual/html_node/Searching-for-Libraries.html#Simple-Library-Tests