oslib-posix: Fix new compiler error with -Wclobbered

Newer versions of gcc report a warning (or an error with -Werror) when
compiler option -Wclobbered (or -Wextra) is active:

util/oslib-posix.c:372:12: error:
 variable ‘hpagesize’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]

The rewritten code fixes this warning: variable 'hpagesize' is now set and
used in a block without any call of sigsetjmp or similar functions.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>