Tab-completion tests require readline support.  The ones in 
gdb.base/completion.exp are already conditional, but the recently-added 
gdb.linespec/explicit.exp tests are not.  This patch fixes that, and 
also an obvious typo ("faill" -> "fail").  The patch is kind of long 
because I had to insert an "if" block and reindent, but there's nothing 
else going on here.  OK to commit?

BTW, independently of the readline-related failures on Windows host that 
are addressed by this patch, on Linux host I've also run into the 
failures already noted in this thread:

https://sourceware.org/ml/gdb-patches/2015-08/msg00432.html

The immediate problem is that the response to the "break -function 
mai\t" test doesn't always include the ^G character in the output 
pattern, and the other FAILs are cascading off of the testcase not being 
structured to recover from a pattern match failure (it's sending more 
commands without a newline to terminate the one that failed).  But, I 
have no clue why GDB is giving a ^G in some configurations and not 
others (I observed that this test fails on nios2-elf target but passes 
on nios2-linux-gnu, for instance), or whether this difference is 
indicative of an actual bug.  So I've not touched that part of the testcase.

-Sandra