RFR: Use SIGRTMAX instead of __SIGRTMAX

[Mikael Vidstedt]

The JDK uses (in two places) __SIGRTMAX, but posix refers to SIGRTMAX without the double-underscores prefix. Also, posix also mentions that SIGRTMAX is not guaranteed to be a constant expression. musl doesn’t define __SIGRTMAX, and the definition of SIGRTMAX is not a constant expression.

This change updates the code to use SIGRTMAX, and also moves the initialization of the static global variable sigWakeup in linux_close.c to be initialized in the function just before the signal handler gets set up.

http://cr.openjdk.java.net/~mikael/webrevs/portola/sigrtmax/webrev.00/jdk/webrev/

Cheers,
Mikael