Use better variable names in MIPS syscall macros.

Use better variable names in MIPS syscall macros.

Carlos noted in
<https://sourceware.org/ml/libc-alpha/2015-05/msg00680.html> that
various ports use potentially problematic short variables names in
their syscall macros, which could shadow variables with the same name
from containing scopes.

This patch fixes variables called err and ret in MIPS macros.  (I left
result_var and _sys_result - separate variables in different macros,
which need separate names - alone.)

Tested for mips64 (all three ABIs) that installed stripped shared
libraries are unchanged by this patch.

	* sysdeps/unix/sysv/linux/mips/mips32/sysdep.h (INLINE_SYSCALL):
	Use variable name _sc_err instead of err.
	[__mips16] (INTERNAL_SYSCALL_NCS): Use variable name _sc_ret
	instead of ret.
	* sysdeps/unix/sysv/linux/mips/mips64/n32/sysdep.h
	(INLINE_SYSCALL): Use variable name _sc_err instead of err.
	* sysdeps/unix/sysv/linux/mips/mips64/n64/sysdep.h
	(INLINE_SYSCALL): Likewise.