i386: Use comdat instead of .gnu.linkonce for i386 setup pic register (BZ #20543)

GCC has moved from using .gnu.linkonce for i386 setup pic register with
minimum current version (as for binutils minimum binutils that support
comdat).

Trying to pinpoint when binutils has added comdat support for i686, it
seems it was around 2004 [1].  I also checking with some ancient
binutils older than 2.16 I see:

test.o: In function `__x86.get_pc_thunk.bx':
test.o(.text.__x86.get_pc_thunk.bx+0x0): multiple definition of `__x86.get_pc_thunk.bx'
/usr/lib/gcc/x86_64-linux-gnu/5/../../../i386-linux-gnu/crti.o(.gnu.linkonce.t.__x86.get_pc_thunk.bx+0x0): first defined here

Which seems that such version can not handle either comdat at all or
a mix of linkonce and comdat.  For binutils 2.16.1 I am getting a
different issue trying to link a binary with and more recent
ctri.o (unrecognized relocation (0x2b) in section `.init', which is
R_386_GOT32X and old binutils won't generate it anyway).

So I think that either unlikely someone will use an older binutils than
the one used to glibc and even this scenario may fail with some issue
as the R_386_GOT32X.  Also, 2.16.1 is quite old and not really supported
(glibc itself required 2.25).

Checked on i686-linux-gnu.

[1] https://gcc.gnu.org/ml/gcc/2004-05/msg00030.html

(cherry picked from commit 35200fd3892f6caf867bf89bc8048e553906af28)