linux-user/sparc: Use WREG_SP constant in sparc/signal.c

linux-user/sparc: Use WREG_SP constant in sparc/signal.c

s/UREG_FP/WREG_SP/g

This is non-obvious because the UREG_FP constant is fact wrong.
However, the previous search-and-replace patch made it clear that
UREG_FP expands to WREG_O6, and we can see from the enumeration in
target/sparc/cpu.h that WREG_O6 is in fact WREG_SP, the stack pointer.

The UREG_SP define is unused; remove it.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20191106113318.10226-7-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>