[MIPS] Don't abort the build process if '-msym32' isn't supported

-msym32 and previously the strategy to tell the compiler to generate
64-bit code but the assembler to put it into 32-bit ELF was initially
a hack to get around the lack of proper 64-bit binutils support and
later turned into a neat optimization with significant code size
savings.  But it's really just an optimization so there is nothing
wrong with just dropping the option (and whatever else goes along with
it, I forgot all the nasty details) on the floor if due to a vintage
compiler it can't be suported.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>