arm64: Fix sig_atomic_t limit definitions

sig_atomic_t is defined as a long and thus is 64-bit on arm64. For some
reason its limit was incorrectly specified as a 32-bit number. This had
the unfortunate side effect of causing gnulib to override most of the
definitions in stdint.h. On CheriBSD this breaks all software that uses
gnulib in annoying and hard to debug ways.

Technically updating the limits might be an ABI change, but these
defines are largely unused (the only use in tree is in the libc++ test
suite where it's use an assertion that will fail due to this bug).
Further, since the underlying type remains the same, we're just
increasing the range of values a paranoid program might use.

Reviewed by: andrew, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D39193