When using -linkshared, the external linker on ppc64x cannot
always handle the direct branch to syscall.Syscall and similar
similar functions when the offset is too far. Instead it should
be done as a BL which can then be called through a procedure
linkage table entry.

This change removes functions Syscall, Syscall6,
RawSyscall, RawSyscall6 from asm_linux_ppc64x.s and instead
creates Go functions which call their corresponding
functions in the syscall package. As Go functions, they can be
inlined with the help of CL 147361.

Fixes golang/go#16662

Change-Id: Ibd2b6ec15b0781c3d7db25e249a3ffc9e1c2884b
Reviewed-on: https://go-review.googlesource.com/c/146518
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>