MIPS32: Make sigset64_t equal to sigset_t

sigset_t is already large enough on mips so use the same principle
as on LP64 architectures: typedef sigset_t sigset64_t

Also prevent accessing sa_restorer field for architectures which
don't define SA_RESTORER in signal_test.

This allows compilation of signal_test.

Test: bionic-unit-tests in QEMU (MIPS32)
Change-Id: I965e202c9a7f7d109e4ec25ad87172dc7c1b8806