Update from Gnulib

Update from Gnulib

This incorporates:
2020-08-23 intprops: be consistent about +X vs X+0
2020-08-23 intprops: fix INT_MULTIPLY_WRAPV bit-field bug
2020-08-23 verify: Make assume work on bit field expressions
2020-08-23 libc-config: Improve comments
2020-08-22 verify: Do use __builtin_assume on clang
2020-08-22 sig2str: Add more signals
2020-08-21 sigdescr_np: New module
* lib/cdefs.h, lib/intprops.h, lib/sig2str.c, lib/string.in.h:
* lib/verify.h, m4/string_h.m4: Copy from Gnulib.
* lib/gnulib.mk.in: Regenerate.