linux(4): Fix the type of a constant in the signal mask macro

Since l_sigset_t is 64-bit unsigned on all Linuxulators, fix the type
of a constant in the signal mask manipulation macro.
The suffix L indicates type long which is 32-bit on i386, therefore,
bitwise operations between a 32-bit constant and 64-bit signal mask
lead to the wrong result.

Pointy hat to: dchagin
MFC after: 2 weeks