libstdc++-v3/libsupc++/eh_call.cc: Avoid warning with -fno-exceptions.

libstdc++-v3/libsupc++/eh_call.cc: Avoid warning with -fno-exceptions.

When building with -fno-exceptions, __throw_exception_again expands to
nothing, causing a "suggest braces around empty body in an 'if'
statement" warning.

This patch adds braces, like what was done in eh_personality.cc in svn
r193295 (git g:54ba39f599fc2f3d59fd3cd828a301ce9b731a20)

2020-09-11  Torbjörn SVENSSON  <torbjorn.svensson@st.com>
	    Christophe Lyon  <christophe.lyon@linaro.org>

	libstdc++-v3/
	* libsupc++/eh_call.cc: Avoid warning with -fno-exceptions.