config, aarch64: Use a more compatible sed invocation.

config, aarch64: Use a more compatible sed invocation.

Currently, the sed command used to parse --with-{cpu,tune,arch} are
using GNU-specific extension (automatically recognising extended regex).

This is failing on Darwin, which defualts to Posix behaviour.
However '-E' is accepted to indicate an extended RE.  Strictly, this
is also not really sufficient, since we should only require a Posix
sed.

gcc/ChangeLog:

	* config.gcc: Use -E to to sed to indicate that we are using
	extended REs.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>