improve --help for --main-stacksize and supported ARM cpu
If the command line option --main-stacksize is not used,
the current ulimit value is used, with a min of 1MB
and a max of 16MB. Document this min/max default formula
in the --help.

Also indicate that Valgrind supports ARMv7


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13426 a5019735-40e9-0310-863c-91ae7b9d1cf9