[MIPS] Remove '-mno-explicit-relocs' option when CONFIG_BUILD_ELF64

This patch removes '-mno-explicit-relocs' usage when
CONFIG_BUILD_ELF64 is set since this option was only required
with the old hack to truncate addresses at the assembly level
where "-mabi=64 -Wa,-mabi=32" was used.

This should yield a small code size improvement for inline
assembly, where the R constraint is used.

The idea is coming from Maciej <macro@linux-mips.org>.

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