package/gcc: disable libmpx for gcc6-based musl builds

package/gcc: disable libmpx for gcc6-based musl builds

Building this minimal defconfig

BR2_TOOLCHAIN_BUILDROOT_MUSL=y
BR2_GCC_VERSION_6_X=y

fails:

In file included from ../../../../libmpx/mpxrt/mpxrt.c:54:0:
../../../../libmpx/mpxrt/mpxrt.c: In function 'read_mpx_status_sig':
../../../../libmpx/mpxrt/mpxrt.h:52:42: error: invalid application of
'sizeof' to incomplete type 'struct _libc_fpstate'
 #define XSAVE_OFFSET_IN_FPMEM    sizeof (struct _libc_fpstate)

To fix disable libmpx for musl builds, other projects did the same:

https://github.com/crosstool-ng/crosstool-ng/commit/3ec2211548a853203e070af6810f49825ec46a6a
http://git.alpinelinux.org/cgit/aports/commit/main/gcc/APKBUILD?id=1830e485126ea9a95d763317fb0c508c1ff297d2

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>