5.0.3 has test failures on riscv64 with gcc-10

[Ken VanDine]

I worked around the failure by forcing gcc-9 on riscv64, but we should solve it properly.

Link to the build failure:

https://launchpadlibrarian.net/494165824/buildlog_ubuntu-groovy-riscv64.cgal_5.0.3-1_BUILDING.txt.gz

Excerpt:

make[2]: Entering directory '/<<PKGBUILDDIR>>/debian/tests'
g++ rounding_modes_simple.cpp -o rounding_modes_simple -frounding-math -g -O2 -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -Wl,-Bsymbolic-functions -Wl,-z,relro -Wl,-z,now
make[2]: Leaving directory '/<<PKGBUILDDIR>>/debian/tests'
debian/tests/rounding_modes_simple
fegetround() = FE_TONEAREST
fesetround (FE_TOWARDZERO)
fegetround() = FE_TONEAREST (error)
fetestround() = FE_TONEAREST (error)
fesetround (FE_UPWARD)
fegetround() = FE_UPWARD (ok)
fetestround() = FE_UPWARD (ok)
fesetround (FE_DOWNWARD)
fegetround() = FE_DOWNWARD (ok)
fetestround() = FE_DOWNWARD (ok)
fesetround (FE_TONEAREST)
fegetround() = FE_TONEAREST (ok)
fetestround() = FE_TONEAREST (ok)
make[1]: *** [debian/rules:56: override_dh_auto_test-arch] Error 2
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:27: binary-arch] Error 2
dpkg-buildpackage: error: debian/rules binary-arch subprocess returned exit status 2