arm/pmu: Fix inline assembly for Clang

Clang complains here:

arm/pmu.c:201:16: error: value size does not match register size specified by
 the constraint and modifier [-Werror,-Wasm-operand-widths]
        : [pmcr] "r" (pmcr)
                      ^
arm/pmu.c:194:18: note: use constraint modifier "w"
        "       msr     pmcr_el0, %[pmcr]\n"
                                  ^~~~~~~
                                  %w[pmcr]
arm/pmu.c:200:17: error: value size does not match register size specified by
 the constraint and modifier [-Werror,-Wasm-operand-widths]
        : [loop] "+r" (loop)
                       ^
arm/pmu.c:196:11: note: use constraint modifier "w"
        "1:     subs    %[loop], %[loop], #1\n"
                        ^~~~~~~
                        %w[loop]
arm/pmu.c:200:17: error: value size does not match register size specified by
 the constraint and modifier [-Werror,-Wasm-operand-widths]
        : [loop] "+r" (loop)
                       ^
arm/pmu.c:196:20: note: use constraint modifier "w"
        "1:     subs    %[loop], %[loop], #1\n"
                                 ^~~~~~~
                                 %w[loop]
arm/pmu.c:284:35: error: value size does not match register size specified
 by the constraint and modifier [-Werror,-Wasm-operand-widths]
        : [addr] "r" (addr), [pmcr] "r" (pmcr), [loop] "r" (loop)
                                         ^
arm/pmu.c:274:28: note: use constraint modifier "w"
        "       msr     pmcr_el0, %[pmcr]\n"
                                  ^~~~~~~
                                  %w[pmcr]
arm/pmu.c:284:54: error: value size does not match register size specified
 by the constraint and modifier [-Werror,-Wasm-operand-widths]
        : [addr] "r" (addr), [pmcr] "r" (pmcr), [loop] "r" (loop)
                                                            ^
arm/pmu.c:276:23: note: use constraint modifier "w"
        "       mov     x10, %[loop]\n"
                             ^~~~~~~
                             %w[loop]

pmcr should be 64-bit since it is a sysreg, but for loop we can use the
"w" modifier.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201001072234.143703-6-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>