We do not want a shell command substitution, but a parameter
substitution (with assignment). Replace $() -> ${}, otherwise
the expanded command return an empty string and the $cross_cc
variable is not set.

Fixes: 634ef789 ("tests/tcg: add more default compilers to configure.sh")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[AJB: disable sh4 linux-test]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211023164329.328137-1-f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211026102234.3961636-9-alex.bennee@linaro.org>