tests:math: fix build of math tests for SH4

Rename libm-test-ulps-sh4 to libm-test-ulps-sh as the ULPS file
is expected to match pattern libm-test-ulps-$(TARGET_ARCH).
Indeed "sh4" is the TARGET_SUBARCH.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>