mips: Use Linux kABI for syscall return

It changes the mips INTERNAL_SYSCALL* and internal_syscall*  macros
to return a negative value instead of the 'a3' register value on then
'err' macro argument.

The macro INTERNAL_SYSCALL_DECL is no longer required, and the
INTERNAL_SYSCALL_ERROR_P macro follows the other Linux kABIs.
The redefinition of INTERNAL_VSYSCALL_CALL is also no longer
required.

Checked on mips64-linux-gnu, mips64n32-linux-gnu, and mips-linux-gnu.