toke.c, S_scan_ident: Ignore whitespace on both sides of ${ ... }

${ var } (and ${ \7LOBAL_PHASE}) were broken by 32833930e. However,
it turns out that the behavior prior to that commit was already
strange:

${ .};              # Legal
${. };              # Illegal
${ . };             # Illegal
${  \7LOBAL_PHASE}; # Legal
${  ^GLOBAL_PHASE}; # Illegal
${^GLOBAL_PHASE };  # Illegal

This commit restores ${ var } and makes all of the above work by
always ignoring spaces on both sides of ${ ... }.