gcc: update compiler architecture to match gcc-runtime (armv6, armv7a)

gcc: update compiler architecture to match gcc-runtime (armv6, armv7a)

The gcc-runtime recipe builds the gcc libraries including libstdc++ with
$TARGET_CC_ARCH flags, which include -march=FOO flags that affect
whether atomic instructions are available.  This causes an ABI
incompatibility when the compiler by default generates code for less
capable architectures.  For example, gcc-runtime libraries on a
Cortex-A8 are built with a different C++11/C++14 mutex implementation
than is used code compiled outside OE and without architecture-specific
flags.

This commit fixes the problem specifically for ABI issues related to
atomic instructions available in ARMV6 and subsequent architectures.
Other ABI incompatibilities may remain in other architectures.

See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62100

(From OE-Core rev: 0ba6ab39f187ecd4261f08e768f365f461384a3a)

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>