x86: Drop _X86EMU_env definition when CONFIG_BIOSEMU is used

x86: Drop _X86EMU_env definition when CONFIG_BIOSEMU is used

With x86 we can execute an option ROM either natively or using the
x86 emulator (if enabled with CONFIG_BIOSEMU). Both of these share
the _X86EMU_env variable, with the native code using it to hold
register state during interrupt processing.

At present, in 32-bit U-Boot, the variable is declared twice, once
in common code and once in code only compiled with CONFIG_BIOSEMU.

With GCC 11 this causes a 'multiple definitions' error on boards
with CONFIG_BIOSEMU.

Drop the emulator definition when CONFIG_BIOSEMU is used.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>