toolchain-glibc.eclass: fix glibc-headers bootstrap on non-multilib host

toolchain-glibc.eclass: fix glibc-headers bootstrap on non-multilib host

The error is found by wm_ attempting to build a cross-compiler
    CHOST=powerpc-unknown-linux-gnu CTARGET=mips64-unknown-linux-gnu
Boostrap failed on glibc-headers (./configure phase) as:

  configure:2623: powerpc-unknown-linux-gnu-gcc -mabi=n32 -c -O1 -pipe -U_FORTIFY_SOURCE conftest.c >&5
  powerpc-unknown-linux-gnu-gcc: error: unrecognized command line option '-mabi=n32'; did you mean '-mabi=d32'?

Note how target's ABI is mexed into host's compiler flags.

The error above happens on every host=non-multilib target=multilib combination.
I've reproduced it on:
    CHOST=i686-pc-linux-gnu CTARGET=mips64-unknown-linux-gnu

The fix is not to inject target's CFLAGS ABI into CC for
USE=headers-only bootstrap step as we don't have cross-compiler yet.

Tested by successfully bootstrapping cross-compiler on
    CHOST=i686-pc-linux-gnu CTARGET=mips64-unknown-linux-gnu

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>