core: add BR2_HOSTARCH_NEEDS_IA32_COMPILER option

Some bootloaders (such as Grub) need to be built for x86 (i.e IA32)
even if the target architecture is x86-64. However, when the target
architecture is x86-64, the cross-compiler generated by Buildroot is
not able to generate 32 bits code.

To solve this, we will rely on the host compiler being a x86 + x86-64
compiler. Therefore, this commit introduces the
BR2_HOSTARCH_NEEDS_IA32_COMPILER option, which tells the dependency
checking logic to verify that the host compiler is indeed capable of
building x86 32 bits code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>