linux-user/elfload: Introduce MIPS GET_FEATURE_REG_EQU() macro

linux-user/elfload: Introduce MIPS GET_FEATURE_REG_EQU() macro

ISA features are usually denoted in read-only bits from
CPU registers. Add the GET_FEATURE_REG_EQU() macro which
checks if a CPU register has bits set to a specific value.

Use the macro to check the 'Architecture Revision' level
of the Config0 register, which is '2' when the Release 6
ISA is implemented.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201214003215.344522-5-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>