AArch64: Fix hwasan failure in readline.

AArch64: Fix hwasan failure in readline.

My previous fix added an unchecked call to fgets in the new function readline.
fgets can fail when there's an error reading the file in which case it returns
NULL.  It also returns NULL when the next character is EOF.

The EOF case is already covered by the existing code but the error case isn't.
This fixes it by returning the empty string on error.

Also I now use strnlen instead of strlen to make sure we never read outside the
buffer.

This was flagged by Matthew Malcomson during his hwasan work.

gcc/ChangeLog:

	* config/aarch64/driver-aarch64.c (readline): Check return value fgets.