x86_64: removes unnecessary syscall (XCC)

It was leftover from 32 bit code.

Copy over the changed file and rebuild your cross compiler.
__ros_syscall -> __ros_syscall_noerrno (XCC)

The *errno_loc parameter was never used by anyone, and this func was
left over from when __ros_syscall_errno() called __ros_syscall() under
the hood.

Since everyone who calls this wants no errno, I just removed it.

Rebuild your cross compiler.  (Copy over the changed files, or just make
clean first).
Kernel support for set_errstr()
Userspace handling of errstr and errno (XCC)

Since not all uthreads have TLS, errno and the new errstr need to be
decoupled from the TLS.

When we're a uthread, we'll use storage in the uthread struct for errno
and errstr.  Otherwise, we'll default to the TLS locations (for vcores).
ld.so will continue to use its own area.  Very early in start.c, before
we call out to uthread code, we'd also use the TLS location.

perror() will print out the errstr, if there is one like so:

"your-message: errno-translated-to-text, errstr"

Rebuild your cross compiler.
Arbitrary kernel context backtracing

We can backtrace any context compiled with frame pointers, given the
program counter and frame pointer.

The existing monitor command 'backtrace' will still backtrace the
calling context, when given no arguments.  Additionally, it will
backtrace any frame, given the PC and FP.

Kernel page faults on x86 will backtrace the faulting context.  Riscv
still needs an implementation of backtrace_frame().

backtrace_frame() will sort of work for userspace too, though the kernel
won't know the names of functions, and you need to compile all of
userspace with -fno-omit-frame-pointer (which we don't do by default).
Fixes bug in KFS new inode creation

Wasn't properly initializing the size field.  This would only matter
when creating new files in KFS, and would only trigger if the memory we
alloc'd wasn't already zeroed.

Something like      $ echo "" >> tmpfile    was enough to cause it, once
the bits lined up right.

The previous commit's backtrace was rather useful in determining this
(without it, all we know is there was a KPF in memcpy).