This libgo patch casts SIGSTKSZ to uintptr before comparing it to a
uintptr value.  This fixes build failures with newer versions of glibc
in which SIGSTKSZ has changed such that the type is now long.  The
build failure is a signed-unsigned comparison warning that turns into
an error due to -Werror.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline and GCC 9 and 10 branches.

Ian