docs: Document usage with Xcode 4.1

With Xcode 4.1 'gcc' is symlinked llvm-gcc-4.2 which cannot bootstrap
other gccs.
See http://llvm.org/bugs/show_bug.cgi?id=9571

On my machine the faulty gcc is
gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)

Document a workaround.

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>

# Super-H specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORTS_BOTH_ENDIAN

## select ARCH_DEFAULT_LE

##

## help The Super-H architecture, as defined by:

## help     http://www.renesas.com/fmwk.jsp?cnt=superh_family_landing.jsp&fp=/products/mpumcu/superh_family/