r366302 broke copy_file_range(2) for small values of
input file offset and len.

It was possible for rem to be greater than len and then
"len - rem" was a large value, since both variables are
unsigned.

Reported by: koobs, Pablo <pablogsal gmail com> (Python)
Reviewed by:	asomers, koobs
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D28981

This flag has been set on startup since 65618fdda0f272a823e6701966421bdca0efa301.
However, This causes some of the math-related tests to fail as they report
zero instead of a tiny number. This fixes at least
/usr/tests/lib/msun/ldexp_test and possibly others.
Additionally, setting this flag prevents printf() from printing subnormal
numbers in decimal form.
See also https://www.openwall.com/lists/musl/2021/02/26/1

PR:		253847
Reviewed By:	mmel
Differential Revision: https://reviews.freebsd.org/D28938

This is already defined by the ncurses headers, so just undef it before
defining it again.

This caused LDBL_MANT_DIG to not be defined and therefore the scalbnl
alias was not being emitted for double==long double platforms.

Fixes:		760b2ffc ("Update scalbn* functions to the musl versions")
Reported by:	Jenkins

arm64 has a distinct exception code for single-step, so we can use this
to detect when an unexpected SS trap is encountered, or when an expected
one is not. See db_stop_at_pc().

Reviewed by:	markj, jhb
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28942

This value should be kept in sync with updates to kdb_frame->tf_elr,
since it is queried by PC_REGS() in several places.

Reviewed by:	markj, jhb
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28943

The main issue is that debug exceptions must to be disabled for the
entire duration that SS bit in MDSCR_EL1 is set. Otherwise, a
single-step exception will be generated immediately. This can occur
before returning from the debugger (when MDSCR is written to) or before
re-entering it after the single-step (when debug exceptions are unmasked
in the exception handler).

Solve this by delaying the unmask to C code for EL1, and avoid unmasking
at all while handling debug exceptions, thus avoiding any recursive
debug traps.

Reviewed by:	markj, jhb
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28944

Mainly link errors interrupts should only be activated on fully linked port,
otherwise noise on lanes can cause livelock. But we don't have error
counters yet, so leave these interrupts disabled.

We could just use a C implementation using __builtin_fabs(), but using
this assembly version guarantees that there is no additional prolog/epilog
code. Additionally, clang generates worse code for masking off the top bit
than GCC: https://bugs.llvm.org/show_bug.cgi?id=49377.

This fixes the RISCV64 softfloat world build after cf97d2a1dab8. That commit
added -fno-builtin to the msun tests which resulted in the first references to
fabs (previously the compiler inlined all calls).

Reviewed By:	dim
Reported by:	mjg
Differential Revision: https://reviews.freebsd.org/D28994

Also updated the logic to use subprocess.run() instead of the old
subprocess.getoutput() which also includes stderr and therefore
can trigger an exception inside Path().exists().

Reported by:	gnn

Instead of copying the code as 00646ca2047305fce32d99edc7a8e6dfd801f3b4
did, include the implementation with the function name re-defined.