Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r232264 by tijl:

Copy amd64 _stdint.h to x86 and merge with i386 _stdint.h. Replace
amd64/i386/pc98 _stdint.h with stubs.

MFC r232519 by tijl:

Do not use INT64_C and UINT64_C to define 64 bit integer limits. They
aren't defined for C++ code unless __STDC_CONSTANT_MACROS is defined.

Reported by: jhb

MFC (part of) r235939 by obrien:

Consitently use "__LP64__".
[there are 33 __LP64__'s in the kernel (minus cddl/ and contrib/),
and 11 _LP64's]

Sponsored by: The FreeBSD Foundation

Copy amd64 _stdint.h to x86 and merge with i386 _stdint.h. Replace
amd64/i386/pc98 _stdint.h with stubs.

Fix SIGATOMIC_M{IN,AX} on x86-64. These are meant to be the minimum values that are allowed in a sig_atomic_t, but it looks like they were just copied from the x86 versions, so these definitions violate the C and C++ specs. Mismatch was spotted by the libc++ test suite.

Approved by: dim (mentor)

On mixed 32/64 bit architectures (mips, powerpc) use __LP64__ rather than
architecture macros (__mips_n64, __powerpc64__) when 64 bit types (and
corresponding macros) are different from 32 bit. [1]

Correct the type of INT64_MIN, INT64_MAX and UINT64_MAX.

Define (U)INTMAX_C as an alias for (U)INT64_C matching the type definition
for (u)intmax_t. Do this on all architectures for consistency.

Suggested by: bde [1]
Approved by: kib (mentor)

<stdint.h> should define WINT_M{AX,IN} independent from whether WCHAR_MIN is
defined. Otherwise first including <wchar.h> and then <stdint.h> leads to no
WINT_M{AX,IN} at all.

PR: 64956
Approved by: das (mentor)

Cosmetic and/or trivial sync up with i386.

Approved by: re (rwatson)

Create a new header <machine/_stdint.h> for storing MD parts of
<stdint.h>. Previously, parts were defined in <machine/ansi.h> and
<machine/limits.h>. This resulted in two problems:
(1) Defining macros in <machine/ansi.h> gets in the way of that
header only defining types.
(2) Defining C99 limits in <machine/limits.h> adds pollution to
<limits.h>.