toolchain: extend musl workaround for recent kernels

Kernels v4.7 and newer[1] require the __USE_MISC macro for libc-compat.h
suppressions to be effective. musl does not define this macros, so add an
unconditional definition of __USE_MISC to libc-compat.h.

In addition, since musl define IFF_LOWER_UP and friends as macros (instead of
enums), define __UAPI_DEF_IF_NET_DEVICE_FLAGS_LOWER_UP_DORMANT_ECHO to
suppress the kernel corresponding definitions to avoid conflict.

Fixes (xl2tp):
http://autobuild.buildroot.net/results/0b9/0b9384c4beaac85f3a17c3245c8a7418c2e2e296/
http://autobuild.buildroot.net/results/657/657bc5687cf01aee38d32d45ba57fb62b2bd56d0/
http://autobuild.buildroot.net/results/bcd/bcd81618de2f745a19c923c66d787b5182bb54aa/



[1] Since commit f0a3fdca79 (uapi glibc compat: fix compilation when
    !__USE_MISC in glibc)

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>