linux: Introduce INTERNAL_VSYSCALL

Add an INTERNAL_VSYSCALL() macro that makes a vDSO call, falling back to
a regular syscall, but without setting errno. Instead, the return value
is plumbed straight out of the macro.

Signed-off-by: Evan Green <evan@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>