toolchain: disable PIE for static build with musl

As mentioned in commit 3c93901b (toolchain: add hidden symbol for PIE
support), support for static PIE using musl requires a gcc patch[1]. Buildroot
doesn't carry this patch. Don't enable BR2_TOOLCHAIN_SUPPORTS_PIE in static
build with musl.

Fixes:
http://autobuild.buildroot.net/results/d19/d19bcfcfb33cc5f5b082b97bbd5852d479823b97/
http://autobuild.buildroot.net/results/91f/91f7c6954c337b03f5ac141050d9b1bee1623376/
http://autobuild.buildroot.net/results/996/996116da9241686110b9525eed08cc5acfd379a6/

[1] https://github.com/richfelker/musl-cross-make/blob/b854a42fc1ea19b8c4ec7104848f9d1a6d7995f0/patches/gcc-6.3.0/0010-static-pie-support.diff



Acked-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>