unix: add syscall trampolines for linux/sparc64

[shalseth]

syscall_unix_gc.go and syscall_linux_gc.go declare Syscall, Syscall6,
RawSyscall, RawSyscall6, SyscallNoError and RawSyscallNoError for every
gc-built unix target, with the bodies supplied per architecture in
assembly. linux/sparc64 has the generated syscall, errno, sysnum and
type files and a hand-written syscall_linux_sparc64.go, but no
asm_linux_sparc64.s, so the package does not compile there at all, and
neither does anything importing it.

Add the file in the same shape as the other linux architectures: jump
to package syscall's implementations where they exist, and trap directly
for the two NoError variants. On SPARC the trap is ta 0x6d, with the
syscall number in %g1 and the arguments in %o0-%o5.

linux/mkall.go already lists sparc64, noting that it is "not fully
supported, but there is enough support already to generate Go type and
error definitions". This is the piece that makes that support reachable
rather than inert.

It is wanted downstream as well: moby/sys#249 adds a sparc64 signal map
and was put on hold by its maintainer pending "x/sys/unix implements
linux/sparc64 syscalls".

Built and tested on an UltraSPARC T4-1 running linux/sparc64 with a gc
toolchain that supports the architecture. The package compiles, and
being able to run its test suite is what turned up the two bugs fixed in
CL 822904 and CL 822906, which account for every failure it shows here
bar one that is specific to my kernel build.

Updates golang/go#55000

[gopherbot]

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