package/netsurf: disallow on archs requiring ABI specific CFLAGS

Fixes:
http://autobuild.buildroot.net/results/67ef520d82ea529a9fe593d83a3aeae5f8b0ee5d/
http://autobuild.buildroot.net/results/eafc3e4be571d5ecee549a11530ac4e508f31782/
http://autobuild.buildroot.net/results/ba7f30833fef54162a82f4b336a72d6599594526/



The netsurf build system mixes up host and target CFLAGS, so it isn't
compatible with architectures where we pass ABI specific compiler flags (in
TARGET_ABI).

Add a _ARCH_SUPPORTS kconfig variable matching the TARGET_ABI logic we have
in package/Makefile.in, and use it to disallow netsurf for those
architectures.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>