linux-user: Move syscall error detection into safe_syscall_base

The current api from safe_syscall_base() is to return -errno, which is
the interface provided by *some* linux kernel abis.  The wrapper macro,
safe_syscall(), detects error, stores into errno, and returns -1, to
match the api of the system syscall().

For those kernel abis that do not return -errno natively, this leads
to double syscall error detection.  E.g. Linux ppc64, which sets the
SO flag for error.

Simplify the usage from C by moving the error detection into assembly,
and usage from assembly by providing a C helper with which to set errno.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>