Fix arm target build with inhibit_libc

Do not declare abort in "libgcc/unwind-arm-common.inc" since it is already
provided by "tsystem.h".  It fixes the following build error:

In file included from libgcc/config/arm/unwind-arm.c:144:
libgcc/unwind-arm-common.inc:55:24: error: macro "abort" passed 1 arguments, but takes just 0
   55 | extern void abort (void);

libgcc/

	* unwind-arm-common.inc (abort): Remove.