jamvm: fix build with uClibc on i386/x86_64

jamvm: fix build with uClibc on i386/x86_64

Commit 0b6b67f34d6d4aeb340bbca579dbf85363c3f4ea ("jamvm: add patch to
fix musl build") introduced a patch to fix the jamvm build with the
musl C library. While the commit log pretends that the build was still
working with uClibc, it is not correct: it no longer builds fine with
uClibc on i386/x86_64, because the Buildroot default configuration for
uClibc doesn't enable <fenv.h> support.

Therefore this commit adapts the patch to use <fenv.h> if available
(which is the case with musl), and otherwise fall back to
<fpu_control.h>, which is available in uClibc.

Thanks to Waldemar for the investigation.

Fixes:

  http://autobuild.buildroot.net/results/325a50d15e1836b31df4e84ba83b296abfb73041/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>