Disable warning when asm.js validation is already disabled.

When ASM_JS is already 2, no need to warn about allowing memory growth
as the user must have known about this and set ASM_JS to 2.
Set ASM_JS default prior to processing command line.

This lets the user set ASM_JS to 2 and avoid a warning if they
also set ALLOW_MEMORY_GROWTH.
Merge pull request #3068 from waywardmonkeys/disable-warning-when-asm-js-validation-already-disabled

Disable warning when asm.js validation is already disabled.