microblaze: Remove support for gcc < 4

Since commit cafa0010 ("Raise the minimum required gcc version
to 4.6") , the kernel can no longer be compiled using gcc-3.
Hence drop support code for gcc-3.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210210141140.1506212-5-geert+renesas@glider.be


Signed-off-by: Michal Simek <michal.simek@xilinx.com>