ARM: pxa: fix building with clang

ARM: pxa: fix building with clang

The integrated assembler in clang does not understand the xscale
specific mra/mar instructions:

arch/arm/mach-pxa/pxa27x.c:136:15: error: unsupported architectural extension: xscale
        asm volatile(".arch_extension xscale\n\t"
arch/arm/mach-pxa/pxa27x.c:136:40: error: invalid instruction, did you mean: mcr, mla, mrc, mrs, msr?
        mra r2, r3, acc0

Since these are coprocessor features, the same can be expressed using
mrrc/mcrr, so use that for builds with IAS.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20221215162529.3659187-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>