package/rustc: fix conditions related to MIPS support

package/rustc: fix conditions related to MIPS support

For MIPS64 architecture variants supported by Rust, we need to exclude
MIPS64R6, but due to a copy/paste mistake in commit
d69d40c029dc7d8199b745eaee759d92b66c5d17 ("package/rustc: add support
for Tier 1 and Tier 2 platform"), we used BR2_MIPS_CPU_MIPS32R6 for
both MIPS 32-bit and MIPS 64-bit configurations, while
BR2_MIPS_CPU_MIPS64R6 should be used on MIPS 64-bit.

Fixes:

  http://autobuild.buildroot.net/8bab232eb98b164df300581ae019254bde7c8ca3/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>