pure-ftpd: disable PIE on ARC

Even though ARC gcc understands "-pie" option and attempts to generate
PIE binaries as of today PIE is not really supported for user-space
applications.

So we disable PIE detection if building for ARC.
That first fixes http://autobuild.buildroot.net/results/57a/57a6180809bc51f7206280533f0b9898ce4cfbb4


and also prevents execution of non-supported PIE binary in runtime.

Interesting enough reported build failure is not directly related to PIE
it looks like if PIE is enabled then successful detection of explicit_bzero()
happens erroneously.

A simple test app that mentions explicit_bzero() while built with
"-pie" gives this output:
--------------------------------->8-----------------------------
arc-linux-gcc test.c -pie
/home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/../../../../arc-snps-linux-uclibc/bin/ld: /home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../arc-snps-linux-uclibc/sysroot/usr/lib/crt1.o: warning: unresolvable relocation against symbol `__uClibc_main' from .text section
/home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/../../../../arc-snps-linux-uclibc/bin/ld: /home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/crtbegin.o: warning: unresolvable relocation against symbol `__deregister_frame_info@@GCC_3.0' from .text section
/home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/../../../../arc-snps-linux-uclibc/bin/ld: /home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/crtbegin.o: warning: unresolvable relocation against symbol `__deregister_frame_info@@GCC_3.0' from .text section
/home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/../../../../arc-snps-linux-uclibc/bin/ld: /home/abrodkin/Tools/arc/gnu/2015.06-rc1-uclibc-archs/bin/../lib/gcc/arc-snps-linux-uclibc/4.8.4/crtbegin.o: warning: unresolvable relocation against symbol `__register_frame_info@@GCC_3.0' from .text section
--------------------------------->8-----------------------------

And what's really interesting exist status is 0!:
--------------------------------->8-----------------------------
$ echo $?
0
--------------------------------->8-----------------------------

With removal of "-pie" we're getting expected failure and exist status 1:
--------------------------------->8-----------------------------
$ arc-linux-gcc test.c
/tmp/ccaWbKwc.o: In function `main':
test.c:(.text+0x1c): undefined reference to `explicit_bzero'
collect2: error: ld returned 1 exit status

$ echo $?
1
--------------------------------->8-----------------------------

We'll look into that strange issue separately internally.
I filed internal STAR 9000925001 for that issue.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>