boost: fix build failures on NIOSII and Microblaze

The fenv support provided by glibc on NIOSII and Microblaze is not
sufficient for Boost, causing the same build failures as the ones we had
with uClibc.

To address this, we adapt (and rename) the existing
0006-uclibc-fenv.patch by forcefully disabling fenv support in Boost on
NIOSII and Microblaze, in addition to the existing uClibc exclusion.

Fixes (for NIOSII):

  http://autobuild.buildroot.net/results/9b8c5d2cd31d63ae70b41d1434868c83e65b8423/

Fixes (for Microblaze):

  http://autobuild.buildroot.net/results/0b9db4c5cfd80d75eb620dbf7a6201faed7230a1/



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