gdb/testsuite: fix FAILs in fileio.exp

I'm experiencing intermittent FAILs in fileio.exp when running on (my)
CI:

    FAIL: gdb.base/fileio.exp: Open a file
    FAIL: gdb.base/fileio.exp: Creating already existing file returns EEXIST
    FAIL: gdb.base/fileio.exp: Open for write but no write permission returns EACCES
    FAIL: gdb.base/fileio.exp: Writing to a file
    ...

The problem turned out to be the way the OUTDIR gets defined in fileio.c.
The path is passed down "naked" and turned into string by STRINGIFY macro.

However, if the path happens to contain name of unrelated pre-existing
C macro, this macro gets expanded during the "stringification", resulting
in (likely) different path than used in fileio.exp and therefore causing
failures.

For example, if the GDB is compiled and tested in directory

    /var/lib/jenkins/workspace/binutils-gdb/build/x86_64-linux-gnu

then fileio.c is compiled with

   -DOUTDIR_=/var/lib/jenkins/workspace/binutils-gdb/build/x86_64-linux-gnu/gdb/testsuite/outputs/gdb.base/fileio

But because there's also C macro named "linux" defined to 1, the resulting
OUTDIR is actually:

   /var/lib/jenkins/workspace/binutils-gdb/build/x86_64-1-gnu/gdb/testsuite/outputs/gdb.base/fileio

This commit fixes this by defining the OUTDIR as string literal in first
place (similarly to how it was done prior commit cc91060) and updating
quote_for_host to handle strings that themselves contains quote (").

Tested on x86_64-linux by running all tests using quote_for_host with
both target board unix and host/target board local-remote-host-native.

Approved-By: Tom Tromey <tom@tromey.com>