POSIX also says that ucontext_t's uc_sigmask has type sigset_t.
MIPS64 strace needs this.
The #define is to keep chromium off our lawn; otherwise it tries to redefine
all this stuff itself. We should probably clean that up and remove the #define.
Change-Id: I765d6a765a33dca7db33cd8c40f408dc98e5c95b